You are having trouble because you are cutting the crown molding in the same fashion as you probably cut baseboard or window or door casing. I can see why this happens. Baseboard trim and window and door case moldings are flat pieces of wood stock with a profiled face. Crown molding looks nearly identical. But look a little closer.
Crown molding has small angled faces as well as a large flat spot on the back of the profiled face.
This is done by design. One of these surfaces is supposed to rest on the wall, and the other one is intended to rest up against the ceiling. Take one of your ruined pieces of molding and cut a 1'-long piece. Use the inside corner of a framing square to simulate the junction of your wall and ceiling. Hold the cut piece of crown molding inside the square to see what I mean.