Ok, I figured out what was up. I'd filed the mag catch so that it was just barely accepting commie mags, and was holding them really tightly. I didnt want to remove too much, but it seems I didnt remove enough, and I guess it must have been holding the mag too high, or at a weird angle or something and was interfering with the carrier coming home. I filed more off the catch and just cycled a mag through and everything seemed to work fine. I guess its better to have those a bit too loose cause its still grabbing some mags really tight.
Anyway, thanks alot guys.