I would be looking at neck tension.
From redding
Inadequate or excessive neck tension can also cause bullet seating depth variations. If you're using a bushing style sizing die, make sure you've selected the correct diameter bushing to size the case necks. Our current recommendation, is to select a bushing tha t is .001" smaller than the neck diameter of your loaded cartridges. (See the bushingselection newsletter in the "Tech Line" section of our website for more information.) As cases are fired over and over, their necks become progressively harder. This can cause the necks to "spring-back" excessively when they are sized, which reduces the neck tension on the bullet. Either anneal the case necks after several firings, or discard the cases and start with new, soft ones.
Heavily compressed loads can create problems when seating bullets. Our Competition Seating Die is not a powder compression die. The excessive force required to seat a bullet on a compressed load can damage the die and may cause seating depth variations. Switching to a faster burning or ball powder may eliminate the need to excessively compress the powder charge.
