The above surface pointed out by @undeadchew is definitely worth checking, the catch should visibly protrude into the mag well. It could also be a damaged mag, if the catch hole on the mag is broken at the edge, it won't hold.
Lastly, a very loose mag well will allow the mag to shift to the side and fall out. A little looseness is normal, but more play than you would find on other (functioning) lowers is not good.
