Thought I would add my $.02.
I purchased my Shield .40 in June, serial # DXP2nnn.
Since then I have put almost 300 rounds through it.

Not a single drop.

Here is what I have noticed; the 6 round mag is very loose. I can wiggle it around when it is inserted with the slide open. I have pushed down on the mag with the slide open, even though it is still loose, it did not fall out.

The 7 round mag is much tighter, and has very little play. Again, I was not able to get it to drop by manually manipulating. And, it as well as the 6 round have never dropped in almost 300 rounds.

This is my first M&P, and one of things that surprised me was the visual low quality of the mags. They almost appeared to be galvanized steel rather than stainless.

My XD mags shine like new penny, and my M22a mags look like quality stainless. The Shield mags do not.

So what is the possibility that it is a mag problem and not a pistol problem.

Has anyone with the mag drop problem tried mags from a buddy that does not have the drop problem?