Have recently bought my first trials bike which is a 2007 beta 250. I have problems with the clutch slipping. When I first bought the bike the clutch wasn't working so stripped it all down and unstuck all the plates. I got told it was a new clutch when I bought the bike and on inspection it does look new. He also said it had a new slave. When I used it for the first time the clutch operates faultless and selecting gears is easy however it is slipping lots. Anyone had this before And any ideas what's wrong? Thanks very much.

Possibly muck stuck behind the washer on the master cylinder, not letting the clutch auto bleed itself, or the adjustment screw on the lever in too far doing the same thing