Duilin, I've done some searching (since I just got my WE 1911 Tactical and was having the slide not-locking-back problem), and it seems you either have to file down the plunger that catches the mag (?), or just use the gun for a while, and it'll eventually wear itself down.
Personally, I am going to wait until it files itself down; last time I opened a GBB, I couldn't put it back together...
