[mehavey]

1. Dirty brass (at least not ridiculously filthy) is not your problem
2. Bullets too big is also not your problem.
3. Crimping while simultaneouly seating IS likely your problem.

Next time:

A. When starting to seat, run a loaded case up in the ram to the top
B. Screw the seating die in until it first touches the case. Back it off at least a quarter/half turn and lock down.
C. Screw the seating (stem) down and adjust to where it seats your bullets to proper depth (nominally middle of crimp groove)
D. Seat all bullets.
E. Unscrew seating stem a half dozen turns to get it well off the bullet
F. Screw the seating die down to where it first touches the (now) loaded case, lower the case, and screw the die down another half turn
G. Now crimp all the loaded rounds (now separate from any contact with the bullet.)