Things that can cause a dragging clutch.

I've long intended to make a video about Ducati dry clutches, but the opportunity has evaded me thus far.  I do have a written "diagnosis" sheet which I'll neaten up and post here so I can link to it easily.  I'll steal some photos from the net to fit the points.  As follows:

If you can't select neutral with the engine running, but it's easy with the engine off, it means the clutch is dragging.  Simple as that.

IME, things that can make it drag, from the top:
 
Air in the system.  Not as hard to bleed as many make out.  If it's hard to bleed, turn the bars to the right and tilt the bike over so that the tip of the lever is the highest point and gently pull the lever in and out.  This will work any air in the hose up to the master and out.  If it's a coffin style master you can spray fluid everywhere doing this, so maybe put the cap on.  Or cover the bodywork, screen, dash, etc first.  Big plastic bags work well.  

You can feel the lever get harder if there's air coming out.  Slow pulls, a few fast, slow, tap the hose, make sure there's no high point loops in the hose, etc.

Some people add bleeder nipples at the master cylinder outlet and bleed there as well.  Never seen the need myself - I just lean the bike over to the right more.  

You can use a syringe to push fluid in from the slave bleeder and up to the top.  Certainly one way to do it, and may dislodge air you can't get to otherwise (possibly more brake based, especially ABS unit).  Just don't make a mess as it comes out the top.

I use a vacuum bleeder to suck the fluid through, as it's faster and works well.  But there's nothing wrong with just opening the bleeder and pumping the lever to push the fluid through.

 
Worn lever pivot.  Either the hole in the lever or the actual pin itself.  With the older style large hollow pin lever pivot, the pins can wear a lot on bikes that don't get lubed, as can the lever.  You can remove the pin, clean and lube and spin it 180 degrees to get a fresh side.  On these masters, the pin pushing the piston can also wear shorter.

On masters with the smaller, solid pin, the barrel on the end of the piston pushrod can wear in the lever, I've seen lots almost worn through the lever.  

The piston pushrod itself screws into the barrel, and underneath there is a grub screw that locks the pushrod in place.  If the grub screw is loose, the pushrod will generally wind itself out, increasing the free play.  Often, with replacement levers this grub screw is loose or not there, so always check that.
 
Conversely, if you want to adjust or remove the pushrod, you need to make sure the grub screw is loose.  It’s a 1mm hex key, so tiny.  and sometimes loctited in, so you might need some heat to make it move.  Can be infuriating.

Crap aftermarket levers.  Either giving excessive free play or no free play.  Might be adjustable, maybe not.  Some of the crap I see is just truly craptacular.  

 
Also, make sure you actually have free play.  And that the piston is returning to the end of its stroke and opening the compensating port.  That’s the little hole closest to the lever end.  B in the photo below.  If fluid can’t flow out of the system, it also can’t get in, and this can cause issues as well when trying to bleed.  If in doubt, give it as much free play as possible, or even remove the lever to see if that suddenly changes the feel, or drops the fluid level.  The fluid should rise a little when you pull the lever in.  On coffin masters, it’ll spurt fluid out of the compensating port, so be careful.

I've brought up the free play thing many times on forums, etc, and lots of people just don't get it.  Or get very offended, like I've insulted their manhood.  Seriously - make sure you have free play.  Add more, just in case it helps.  If I have one that just won't bleed, I often remove the lever to make sure.  Even if it was fine last week - add more free play.  You'd be amazed how many times it just works.

This is the adjuster on a big pivot lever.  From the factory they often had the flat blade screwdriver slot in the end of the adjuster filled with glue so you couldn't adjust it and mess it up.

Aftermarket clutch slaves.  Usually have a larger in piston diameter, increasing the hydraulic ratio to soften the lever, but decreasing the slave piston travel correspondingly.  It's a ratio trade off.  If the slave is a lot larger, the pressure plate might not be lifting far enough.  Move the lever out, pull it in further, etc.
 
While not really drag specific.  Worn pushrods, slave pistons and seized pushrod bearings in the pressure plate.  The seized bearing will spin the pushrod and it'll wear away and into the piston.  I had a 900ss in once that the owner had had long time clutch issues with from new (tip - it's not the bike, it's you) when it was maybe 15 years old.  It needed a new pack and once fitted I pumped the lever and the slave piston and pushrod were so worn that the piston popped out of the slave body.  Scared the shit out of me when it went bang.
 
Warped steel plates.  There should be 1 x 1.5mm "spring" steel plate, which is conical and if you put it on a flat surface and press the outer edge (if it's the right way up) the opposite edge will lift.  The 7 friction plate pack pre 1100/1098 models had would also have 8 x 2mm flat steel plates.  The thickness of those packs from Ducati is nominally 38.5mm.  The 1100/1098> packs with 8 frictions and 10 steels will usually have some 1.5mm flat steel plates to reduce the overall thickness due to the extra plates. From memory they're around 40.5mm, but done hold me to that.  Pack thickness itself, as long as it's not stupid either way, isn't an issue IMO.  Less thickness reduces spring preload and thereby lever effort.  More thickness and the friction plates might come out the end of the basket and not go back in.

Using a very flat surface, check the rest of the steel plates to see if any of the other plates behave like the spring plate, flipping them to check both sides.  If they do, they're warped and need replacing.  You can't get around them being warped.  You may also find someone has fitted more than 1 of the 1.5mm spring plates for some reason, valid or not.  There was a DP slipper for 1198 that liked 4 of them.  Really don't understand how it worked without dragging like crazy, but I've done it, so I know it was fine.

As above, there are also flat 1.5mm steel plates, used in the 1098 onwards 8 friction plate packs and the Surflex sintered 8 friction plate packs that used to come in SP / SPS/ R models, so don't think any and all 1.5mm steel plates are springs.  Unless you have a stash of good steel plates, the only place to get them is a new complete pack.  OEM, Barnett, Ducabike, Newfren, Adige and Surflex all make packs.  Aluminium plates in a steel basket will make the plates sacrificial - it's what I tend to do these days.
 
Hub wear  - the centre part the plates go over.  Watching from above, pull the lever in and see if the spring caps move outwards as the lever free play takes up, then stop and then the pressure plate starts lifting off the pack.  If so, the large star like washer at the front of the hub is eating its way into the hub.  Pretty common.  You can add another washer (i've seen up to 4) or replace the hub if it's very worn.  The original hub centre is a rubber block cush drive assembly, so it will move a little when ok, but only a little.  If you remove the pressure plate and pack and push the hub backwards you can usually see the wear there.

 
Basket and hub groove wear.  I really don't agree that wear in the basket or hub grooves makes any real difference to how the clutch disengages.  At all.  I have a customer who bought a bike that some idiot had put steel plates into an aluminium basket just before he bought it, and 50,000km later the plates had eaten their way quite a few mm into the basket.  It was all absolutely rooted, but still perfectly functional and lovely to use.  Loud as hell though.

Aluminium baskets that have had aluminium friction plates in them will have a burr on the inside of the basket grooves where the basket has been peened by the plates.  I usually scrape that off - it comes off fairly easily with the carbide scraper.  Not sure if it really makes any difference.
 
Sometimes, the peened out bits of the tangs on the friction plates as in the picture below can get between the steel plates and hold the pack apart and make it slip.  Just grind the peened out bits off and stick it back in.  

To diagnose.  Start at the top and work your way down.  It might be a combination of issues, so don't stop until you've checked it all and fixed every problem you come across.