First of all I seat gas checks with a home made in line seater. Check goes in bottom, bullet in top and top punch from top seats check with a couple of hits from hammer. Checks are in line and normally fully seated. After going through base sizer (push through) they are powder coated. Bullets today were cast in a relatively new mold. A number of gas checks popped off even though checks and bullets were dried under outlet of dehumidifier and preheated before check application to eliminate moisture as an issue. I had originally though it was a perfect fit between top lip of check and bottom of first driving band. Found that top of check was actually resting against bottom side of driving band instead of base of bullet. Just another thing to check. Checks were factory and mold was Lee.