Well, I just purchased a pre-ban lower which is cast SN 92XX and should be getting it towards the end of next week. I will test it with some Colt 20 Rd mag and post results. Sure hope it's not a problem. DPMS will probably stand behind it if there is, but what a pain in the a** it will be to have it re-manufactured.
BTW, what kind of mags were the ones that wouldn't work? Were they the cheap USA, non-USGI? This could have been the problem, not the lower.