Simple tests are loading one round into the Tapco mag and firing it, seeing if the bolt locks back. Mine was locking back, but it was on the bottom lip of the carrier, not the bolt face like it should be. If this is an intermittent problem might have to do it a few times to make sure it's operating right. This is the easiest way but not always 100%. Ammo is all over the place on pressures, I always try three or four different ones if I run across any problem to rule out manufacturer shortcomings.
I tracked mine down through a Qtip test, carbon on a black barrel can be hard to see sometimes, lol. Mine actually was a roll pin that was too short, when I put it together I drove the pin in flush against the side of the gas block, but it was too short to go into the opposite side, so the tube twisted a tad, which misaligned the holes on the tube to the barrel and reduced the amount of gas let through. Sometimes it would run fine, I guess the tube could twist a tiny bit, enough to let the bolt run as it should but eventually after a few rounds it would start being a PITA. I'd just take a close look at it and if you don't find anything obvious then just keep mags separate for it, you could always use it for parts for the PSA one if TEOTWAWKI comes knocking...