Most aftermarket mags use thinner metal and poor assembly tolerances, often leaving too much room in the mag so that the follower has too much side-to-side movement.  This will prevent the follower from keeping the last round snug up under the feedlip where it needs to be to feed smoothly.  Also, I've seen a lot of aftermarket mags welded with one mag half higher than the other, which brings one feed lip out of alignment.

This is why I recommend ONLY USGI mags.  It doesn't matter at the range, but never use an aftermarket mag for anything important.

-Troy