I had a Forster Case Trimmer w/the same issue. Drove me mad! Cases were getting trimmed shorter and shorter as I went. Didn't matter how tight I tightened the set screw, it would never hold. Wasted too many cases with this device.
Ended up going with the Lee Cutter & Lock Stud system using the cartridge specific case length gauge (pilot.) Chuck it up in my cordless drill and go to town. Every case comes out the same, uniform length. Simple, accurate, cheap. One of the few things Lee makes that I truly appreciate.