1. Crimp does not retain the bullet in the case as well as neck/ case wall tension itself.
2. Too much crimp will begin to buckle the case immediately behind the case mouth. This will pull the case wall away from the bullet shank. Now you have no more tension and only the crimp to hold the bulet, which is now free to move forward under recoil.
3. What OAL? If you are loading them to max length, then you may have too little powder for too much case volume, and the powder is detonating.
I have a Colt Python that detonates 2.0 grs of Bullseye in a 357 case with 148gr. HBWC bullets flush seated. The cases come out of the cylinder looking like new, complete with the original shine. A heavier charge, like 3.0grs, soots up the chamber and the cases come out nearly black all over. Also, the 2 grain load pirerces primers 75% of the time.
Is that what you're seeing?