Another cause could be the actual clutch plate. When we were first forced to use non-asbestos linings, the early type type linings were made of a smooth grey compound - this worked OK but gave off a fine powder that eventually gathered on the six pins through the flywheel, causing the pins to drag and tip the pressure plate, with the resultant inability to completely free. Later generation linings were made of a woven compound (like Ferodo) and dont seem to suffer in the same way.
Regards,
Eddie.