I found my lockback problem was caused by light ammo and a new rifle. The action on the G2 DPMS was a little stiff out of the box. It would fail to lock back on the last round with 147 gr PMC but worked fine with my 175 gr reloads. I have about 75 rounds through it now and it is locking back fine. The Lancers are really nice mags and I won’t use anything else in the G2. The Pmags won’t function at all in my rifle, the rounds sit too low and the bolt either completely fails to pick them up or barely catches resulting in a stove pipe. All of the DPMS steel mags had to have the lips bent a little for them to function.
Save yourself a lot of grief, get the Lancers you won’t be sorry!