I have a M&P40 I just picked up 2 weeks ago. I have the same issue with the magazines. Now from what I understand, there has been several production changes. Changes to the slide catch, followers, and the trigger bar. All which seems to have some effect with each other.

With the slide catch:
20130921-M&PSlideStops_3Gen | Modern Service Weapons

On Gen 2 & 3, the "tang" seems longer, which is supposed to sit on the magazine follower shown in this picture:
Tapatalk

Now on this site, it lists all the latest production updates:
Current production S&W M&P's (Update)

I believe that since all the latest updates are not installed together it may be causing the magazines to get stuck. Each change created a domino effect. Just my opinion and my rant...Because this is my very first pistol I ever owned and shot. And I suck very badly! HAHA!!

Or you can try this fix:
How to fix the magazine drop free for M&P 9 - YouTube