Can you describe the symptoms in more detail? For example do you get a firm snap when pushing the clutch over center? Does it stay engaged or ever pop back out? Has any parts been replaced? How far have you (or anyone else had it apart?
Sorry for all the questions, but the answers will help pinpoint the problem. Likely it is worn clutch dogs as these are almost always overlooked when rebuilding a poor performing clutch. Although it could be the inner clutch facing if give never had the driving disk off the crankshaft. (You'd be surprise how many young timers don't know there is a facing back there, and only replace the outer ones.)