I've had FTF's in various GI (and other) firearms due to worn magazines. The mags may look generally okay, but on close examination, the cut for the mag catch in the mag body may be worn, which will directly cause FTF's. Additionally misplaced mag lock cuts will cause FTF's. Make sure your test mag(s) are quality ones, hopefully ones that run 100% in other M-1 Carbines.