I literally spent half a night this week trying to diagnose a factory built PSA upper via text. Failure to Feed. Gun was a one shot wonder. Rounds ejecting but not feeding. Suspicion: Gun is possibly WAY overgassed or the gas block is misaligned. But without getting hands on, I can't really say what was wrong with it.
Wonder if it suffers from the same BCG issue I mentioned earlier?
There is the possibility the gas port is obstructed from a poorly drilled hole as well. Although, a snap cap in the chamber and blowing in the muzzle end should be a dead giveaway if gas flow is restricted.