Odds are 99% your FL sizing die is not resizing the brass properly right in front of the belt. You can probably confirm this by measuring any virgin brass you have (right in the front of the belt) then a piece of brass you think is "junk" and if the junk brass is .004 or so larger than a virgin brass you have found your problem. The .004 is an estimate, the number could be a little bigger- but probably not smaller. Bottom line is your FL sizing die is not fully resizing the brass right in front of the belt; very common issue.
The fix is normally a "body die" or custom FL die from Whidden, etc.