Maybe the dies need a little adjusting,and should try using small base sizing die.
That was the solution to my AR-15 and a shooting bud's legal M-16A1. The brass' shoulder likely needs just a very slight push. Be very careful if you crimp with something other than a Lee Factory Crimp die.
As a possible temporary fix. Lower your sizing die and jam the shell holder up real hard against the bottom of the die when re-sizing. It might and I emphasize "might" help. Also using say for example using a RCBS sizer and a Lee or other brand brand shell holder or visa versa could be a possible
If you have a RCBS sizing die. Send your die along with three once fired, never previously resized brass cases. If a small base die is the solution, RCBS will send you another sizing die free of charge.
Better back track, thats what RCBS did for me about 5 years ago, hopefully it is still the same policy. Enclosed with the new die I got was a note warning about crimping causing the shoulder to swell. My friend with the M-16 also did the same with a 308 die he used for his M-1A. Got a new small base die from RCBS no charge.
If you're using a Lee die, forget it. Lee will send you a form letter claiming their die is within SAMI specs. Never had a sizing die problem with Hornady. I did have another problem with one of their expander dies and they promptly sent me a replacement.