Look at the belt seat if it slipping and open the hood or take it off jack it up run it and watch the clutches to see if they both are working the one on the engine the drive clutch will move in when you rev the engine up and the driven will move out.The driven may be stuck on the shaft try turning the two halves in opposite directions or pry it open wont hurt to lube it either.The other thing you need to check is the chain case oil and chain.