Loose neck grip could be your problem. If you are using lubricated lead bullets, I've seen the air compressed in a case when seating a generously lubed bullet partly push the bullet back out of a loose neck grip. Usually this happens on thinner necked cases (e.g., Remington, IME) that don't size down as far to grip the bullet as firmly as thicker necked cases do. Compressed powder could assist the air in this.
To tell if you have this problem, find a round that measures significantly longer than the others after seating, but before crimping. Set it on the bench and push down on its bullet nose with your thumb. If you can seat the bullet deeper with your thumb fairly easily, then you've got the loose neck grip problem.
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