i believe the term your looking for on the type of clutch this is is called a "wet clutch" or "oil bath" clutch.

a couple things come to mind here on this issue
1- is the linkage worn any? could be not getting enough travel
2- usually when one wont go in gear or is hard to shift like this it is a synchronizer issue in the gearbox. usually worn out and wont stop the gear cluster from turning so it can go into gear.

first off I'd double check the adjustment on the clutch linkage and make sure its moving the clutch mechanism, it doesn't have to move far to disengage the clutch disc. since it is oil bath there is little worry about friction wearing the disc out since it is wet all the time.
listen to it when you depress the pedal and see if you can hear it dragging/rubbing.
try testing it with the engine not running, push the clutch and then turn the engine or transmission and see if it is truly disengaged.
maybe you've done this already?
just throwing out some thoughts here :o)