Clutch Failures can often be identified by noting that the engine will rev up but the vehicle will not be propelled forward. This is an indication of a worn out clutch disc or pressure plate. It is our recommendation that the pressure plate, Disc, throw-out bearing, Slave & pilot bearing all be replaced when servicing the clutch. These components go through considerable movements daily and all will deteriorate and be in need of replacement at some point as well.
Other possible things could be having oil on the disc from a leaking engine seal