Could also be down to the wrong type of grease used on the input shaft and on the hub splines. In the aftermarket, all the clutches we sell will have a sachet of grease in the box. This grease is specially formulated for SACHS which also has a dust repellent in it. A lot of people still use copper grease, which has a low melting point, and no dust repellent in it. When the bellhousing temps get high, the grease breaks down. Usual working temps in the bell housing are between 90-110 degree's C. Copper based grease starts to break down at about 180 degrees C i think.
Anyways, getting to the punch line, without the correct grease being used, with the clutch dust, the grease will turn into a thick paste, which will prevent the plate from sliding freely on the input shaft. This could then cause the disengagement issues.
Other things could be damage to the input shaft splines preventing free movement. Sometimes if there has been a gearbox misalignment on the clutch, you get burrs develop then causing the issues above.
The job of the clutch cover is to transmit torque from the engine (this is achieved by the metal straps on the side of the cover) The diaphragm fingers give you the clamping force needed to clamp the drive plate secure between the pressure plate and the flywheel. The metal straps also give you the pressure plate lift to be able to disengage the drive.