I got my 40 shield 2 weeks ago and the first day out I had the problem with the extended mag dropping. The gun ran perfectly with the shorter mag.

I then used the blade of a screw driver to bend the upper lip (where the mag release plunges
into the mag) outward. I believe this gave it a little more surface area to catch onto and it has been perfect ever since. I have fired Federal, Ultramax, Remington JHP's and Remington Golden Sabers and have not been able to get it to drop a mag again through 200 rounds.
The mag still does drop cleanly and freely when you press the release.
I am not saying this is the final solution or even the right one but just wanted to share my experience. I went from a new gun that dropped a mag on every shot to one that fires everything smoothly and accurately.

I really, really like this pistol.