Dan, without having the measurements on the dies and the chamber it is hard to diagnose properly. It does sound like the chamber is to tight for the brass. It is a common misconception that the die sizes the brass all the way down, but it cannot size the part of the casing that is solid brass at the base. Under high pressure loads the base can slowly swell a few thou and if it swells to the point it is tight in the chamber you cannot fix this except to polish out the chamber area to get a little relief.
When you have a tight piece of brass, measure the base before and after resizing and it does not change. If you measure just above the base before and after resizing, you will see a difference about 1 thou give or take a half thou.
It is also a good idea to check the new brass when you get it in and check against your chamber dim., the chamber should be 2 -3 thou bigger or it is to tight for your new brass.