In my opinion (which i am still kinda green to heli's as well), it could be either the one way clutch bearing is somehow messed up and locked onto the mainshaft, or it is the clutch itself, with possibly a sprung clutch or even the clutch shoe is broken. I bet if you drop the engine you will get your answer. It only takes 5-10 minutes to drop the engine out, better to be safe than sorry!