Sounds like the OAL of the case is a little short? Round gets fired, case moves forward in the chamber. Primer backs out a little before the case has a chance to form to the chamber. Pull the bullets out to a jam "into" the lands. Once brass is fireformed to the chamber, no more primers backing out. (been there, done that)
Could also be loose primer pockets. Fireform first and double check headspace with factory loaded ammo. Make sure you're not pushing the shoulders back too far when resizing. (causing the same problem again)