You are oversizing your cases for that chamber. Setting the shoulder back when resizing will cause this problem. If the headspace is even a tad too long, it will exacerbate the issue. Have the headspace rechecked, and if it is good, back that FL die off a bit so you are not setting the shoulder back. That is why neck sized cases work Ok, the shoulder-to-base dimension is not altered by neck sizing. Regards, Eagleye.