1) check/flush clutch fluid. if it is/was low, you may have bubbles, which wont activate the slave cylinder enough to disengage the clutch

2) check for proper master/slave cylinder operation. if you have old parts, and a stiffer pressure plate, these parts may fail.

3) possibly a busted pivot bearing, but more likely 1 and/or 2

4) broken pressure plate teeth. only usually happens with REALLY high pressure loads tho, so unless you are running a rediculous PP, check 1 and 2