Assuming that you have confirmed that the slave is pushing enough to disengage the pressure plate a few things I can think of.
*Clutch plate is stuck on the gearbox input shaft splines (rust?, worn?)
*One of the clutch plate springs has fallen out of the clutch center (broken) and wedged its self between the clutch and the flywheel or the flywheel and the pressure plate).
Cheers
Wilbo
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