Difficulty in selecting neutral is the result of the clutch plates still dragging after the cluch lever is pulled in. As the plate friction material wears, the plate stack height gets reduced so the problem usually becomes less severe over time. If you've recently replaced the clutch plates, the stack height may be too high, which will also cause the problem.
Two possible solutions:
Your 4-position clutch lever adjuster is placing the lever too close to the handlebar so you're not getting complete separation of the plates. Try a different adjuster position that gives a longer lever stroke.
If you are still unable to engage neutral, the most likely cause is air in the clutch hydraulic line. A trapped air bubble can compress (the fluid can't) so you wonít get a full stroke of the clutch pushrod and the clutch doesnít fully disengage.
Air bubbles rise to the highest point in a hydraulic system so that's where bleed nipples belong and are usually placed. In Ducati superbikes the bleed nipple is placed at the lowest point in the system (duh) so the only way you can bleed all the air out is for the bubbles to be carried along with the fluid as you bleed, and bleed, and bleed the system. It is possible to bleed the system without using a bleed nipple at the master cylinder, but sometimes it takes a lot more attempts.
So, the Ducati clutch line is notoriously difficult to bleed using just the nipple at the clutch slave cylinder. Evoluzione and Yoyodyne, two aftermarket manufacturers of slave cylinders, often encountered customer bleeding problem related complaints so began selling a banjo fitting having a bleed nipple for use at the highest point in the hydraulic system. It works.
Hereís what it looks like:
Another common reason that the clutch won't completely disengage is that the aftermarket force-reduction slave units (and later model Ducati slave units) move the clutch pushrod less distance - a design trade-off to accomplish a reduction in clutch lever forces.
Is your slave cylinder an aftermarket unit?