Search:

Are the dies pressing firmly on the shellholder when you resize?
My only other guess would be the problem is a donut/thicker brass in the lower part of the case neck. All same brand, or is it in...

Was the problem caused by a faulty sizing die, or "operator error"?

Maybe bullet seated too long and hitting the lands before case is fully chambered?
More likely a brass size issue though, e.g. chamber at the small end of normal specs, cases at the upper end.

I...

I wonder what it would do to the neck, trying to squeeze it smaller with a projectile in it.
A shoulder bump might be okay, but if it turns out the cases are over-length like Max had they may clamp...