I just finished putting together my formerly felonious weapon. It is a Bushmaster M4 Flattop. All looks good except that the slide will not lock back with a mag in it. It will lock back if a press up on the mag as I let the slide forward but if I don't it won't stay back. It will lock back with green followers though just not black follower, GI, 30 round mags. Any know what the issue is? Thanks!

Not familier with a 'slide' on an AR-15.

Do you mean the bolt carrier is not locking back on an empty magazeine with black followers?

Do these magazines lock the bolt back in other ARs?  If so the problem could be your magazine catch isn't tight enough - press the button all the way and rotate the catch on the other side 1 turn.

If the magazines are untested - they could just be bad magazines (it happens).