My full size 40 M+P, was purchased Feb/March 08, and as a left hander, it was much harder to release the slide stop left handed. However, with my left hand index finger, it would release from the left side of the pistol, no problem.
So,
I literally kept working it and working it. The actual "stopping catch" is on the left side, and with the wrap around slide stop release to the right side, I found a pushing in and down was helpful. You can see how high it catches in the slide from the left side of the pistol, how much does it take to release it?
Manually sliding the slide back, lifting the slide stop, and repeating seemed to really help.. Removing my slide, examining the slide stop, I have a small area of the black finish removed from the catch. You may just need to have a hair taken off to loosen the release from the right side.
When was yours purchased? How many rounds? I have an MPZ serial prefix. I expect they will it up around the slide stop on the left side and maybe even replace the slide stop itself. I don't know about the turn around time for the M+P.
Could an option be a local gunsmith? It sounds like a minor fit and finish kind of thing?