Are you using too heavy grade of oil, which would cause the clutch plates to stick together.
If i were you, i would take all the plates out, wash them off in petrol and lightly cleaning off the bonded plates with wet and dry.
Check that the clutch centre grooves where the plain plates lugs slide in are smooth with no ridges and the samr with the clutch chain wheel.
If there are signs of wear from the plate lugs, try smoothing them out with a file, so the lugs run smoothly in the grooves.
Re-insert the plates and tighten the spring loaded screws until the thread is just about flush with top of the domed nuts.
Slacken clutch cable right off, and adjust the push rod adjusing screw until it just touchs the push rod and then back it off half a turn. Hold screw and tighten lock nut, finally taking up the slack of the clutch cable at the clutch lever end, leaving just a little bit of slack.
Before putting in the correct grade oil, fire it up with the cover off and try the clutch to see if it disengages. if the plates do not free up, check the clutch push rod for length, because the ends may have worn and not give enough throw on the clutch to make it disengage.
