It can start slipping because the disc is worn out. It can also start slipping because the pressure plate pressure is weakening, which can't be seen. I had a JWT clutch wear out prematurely. First, I'd say to figure out whether the clutch is adjusted properly. If it is, just take it apart. If it is not, then show them it is not, point out that it is slipping now, and state that they are responsible for the clutch wearing out prematurely. They may or may not take responsibility.