Could it be that your mag loading stroke was bringing the gun DOWN onto the mag vs. pushing the mag UP into the gun?

I reload with the gun at chin height so I can see it and the target. The gun stays still. I rip the empty mag out with my off hand, drop it or stow it, retrieve the loaded mag and smack it up into the gun. The slide auto-forwards immediately. I acquire a two handed firing grip and go back to work.

If the slide does not auto-forward, my off-hand (Left) thumb hits the stop tab to release it on the way to acquiring the firing grip.

I use the hand-over sling shot method for all malfunction procedures.