I am betting it is a simple fix but here are the symptoms.
I was in an AR class this weekend. I was using the mag that came with the rifle and 11 PSA D&H mags. Day 1 went good, all mags I was using would lock into the mag well with the bolt closed, meaning if I switched mags before running dry, the mag would lock into the well. On day 2, we were working on reloads and suddenly the mags were not locking on a closed bolt, no matter how hard I or the instructors pushed. This was with 28 rounds in a 30 round mag. This became the case with all of the mags I had, no lock on a closed bolt.
We tried backing off the mag catch 1 revolution, advancing the catch one revolution, no change in status. We tried Pmags, they worked. We tried I think possibly Colt mags, they worked. Unsure of what else we tried. But with a few donations I ended up with enough workable mags to complete the drills. When I got home I fiddled further with the mag catch, completely removing it, inspecting and re inserting to flush with the catch and button.
I had no hiccups with the steel cased Wolf .223 ammo I was running all weekend. I just loaded up one suspect mag with 5.56 brass. The mag locked on the closed bolt with 5.56 brass. So I tried more steel cased .223. It too locks on the closed bolt. But I really need to push hard to make that happen, far harder than I did earlier like on day 1 before this malfunction stuff began on both of these mags. It was HOT this weekend, 2 of the hottest days of summer. These tests I am conducting are in my home at temps of 77*F with lowered humidity.
I am suspicious of the mag catch, like the part may be out of spec just enough to not allow the mags to catch reliably. Open for suggestions, gun was pretty much unfired(1 mag thru it) but in my possession since 11/2014.