It sounds like you might be overdoing the case mouth belling step. The case shoul only be flared just enough to allow the base of the bullet to enter the case without shaving lead from it. If this step is overdone, you will create a ring in the case just below the crimp that's too large for the case to fully enter the chamber. Try experimenting with the case flaring a bit. I once had an expander reduced in diameter a couple of thousandths to help create higher bullet "pull", and reduce the amount of flaring a bit.