Have a new upper, put it on an existing lower, first 8 shots fired then only a "click", charging handle was almost impossible to pull the bolt back to eject the unfired round. unfred case shows nothing or a very lite dimple on the primer from the firing pin. Tried this on five or six rounds and no firing only the "click" of the hammer release. Rounds are(primers) fine, they are not the problem. Is it the bolt carrier, or is headspace off and the round is being pushed forward upon firing. Any ideas or similar experience would be grealy appreciated to correct this problem.