What happens when you load ?

Are you shooting fixed or breech seating?

It may actually be a diameter problem, possibly out of round. So careful measuring of cases that have a problem compared with those that don’t.  If case mouths are not expanding the same, it might make sense. Or not all having the same trim length. Either way your probably only a few thousandths difference.

Something else to make sure of is that the ones that remove easily do not have a case mouth that is rolled over. This would be too long for the chamber and being crimped. If that is happening, it might give you the clearance needed for easy removal.

One way to solve the problem would be to remove a little off the rim of the case. This would also give you a way to index the case/cartridge into the chamber each time, in theory increasing accuracy.

The problem may be by removing too much of the rim, the extractor has less to push against. So you would want to rotate the case to fire.

My guess is that you find some inconsistencies in the brass and can trim accordingly without loss of accuracy.

Thanks for the replies. Most helpful.

Thanks again. It seems that the key is to place the hammer on full cock before lowering block.