I've been having the same kind of problem with my new .45 Shield.

It was flawless for the first 6 or so magazines run through it. Then it started malfunctioning. Nothing had changed. Same original two magazines. Same ammo. Gun clean or dirty didn't matter. Happened with both mags. So I tried some different ammo. Made no difference.

I could not get through either mag without at least one failure, usually several. The 6 round mag seemed to be worse than the 7 round.

The problem was after firing a shot, the slide would stay back, but I looked at it carefully and noticed that it wasn't all the way back and the slide lock was not engaged. If I pulled the slide back and let it go, it would load the next round and fire ok after that until the next failure, which was sometimes the next round.

I looked at it even more carefully and noticed that the cartridge in the top of the magazine was not fully up into the lips at the top of the mag. I don't know how that would cause the slide to stop part way back, or come forward part way and stop.

So I took both mags apart and cleaned them thoroughly with Rem oil. Without cleaning the gun, I went out and ran both mags full through without a hitch. It will take some more testing of course, but I'm encouraged by the initial results.