another idea I read here is that the chamber could be out of round, but that should effect every cartridge you shoot. You could check it by indexing, place a ink mark on the side of the brass, load it with the mark at the top, fire, remove, try to insert the brass rotated 90 degrees to the fired condition.
Also, check the dimensions of the fired brass, how much growth in the diameter are you noticing. Could be the brass is not allowed to spring back much, but I dont know if we typically measure well enough compared to the amount of spring back we see in a fired case.