Two other thoughts: have you compared the lengths of the cases as they come out of the sizing die? If you are applying a crimp and set the crimp shoulder up with a short case, longer ones can over-crimp, and that can bulge the case.
The less likely thought: I once bought some foreign-made 45 FMJ ammunition. The bullet diameters were all 0.449" instead of the usual 0.451". Why they made them that way, I don't know, but they also had very thick case necks that brought the mouths of the finished cartridges to SAAMI maximum for a finished cartridge case mouth, which is 0.473". When I subsequently tried reloading them with a 0.452" cast bullet, the cases were at about 0.477" OD, which is 0.001" over SAAMI maximum chamber diameter at the mouth and could not be chambered. If you have a similar problem with odd brass, it would fit your symptoms. Measure the ODs and see what you get.