Could be a number of things. Check magazines first. Does the follower extension catch the slide stop knob of the slide stop? If they slip and ride under, the issue could be an out of spec slide stop or your mags. If the follower in wallowing around too much inside your mags, the follower may be missing your slide stop. There is a possibility that your magazine well is out of spec, that is too wide; ugly possibility.

Exactly how do the mags not catch?

Jack