When M&Ps first came out, people complained about how easy it was to release the slide by ramming the mag. On a later release they changed the slide stop and it was much more stiff. I have an early M&P that I had to send in, and they ended up sending it back with a brand new slide. When it came back it was so stiff you couldn't reasonably release the slide with your thumb. I was disappointed because I always ram the mags in when reloading. Anyway, after a while it loosened right up. I just tried it and was able to release the slide with my thumb just fine on my 40c.

tl;dr if it's a new gun, break it in a little.