I have a .40 M&P with the polymer FSS kit.  It dry fires fine, it works fine with dummy rounds in the mag and cycling through them.  Today I got to the range, and inserted a loaded mag, pulled the slide back and the trigger pulled straight back and nothing happened.  Tried hand cycling a new round, and nothing.  It is as if the slide was off and you were just pulling the trigger back.  It goes all the way back, but no contact with anything.  I got it to fire once, and then nothing afterward that no matter what I tried.  The trigger pulled back but its as if nothing was getting contacted.  if I dry fire it it works fine.  I'm stumped...

It sounds like it needs the trigger loop opened up.

Ok. I watched the video on it.  Seems like it might be the problem.  Will try to open it up a bit and see.

Odd how it works with everything but real rounds in the gun...

If you notice with a full mag. It pushes the slide up.

That's kind of what I was assuming after reading your post and watching the video.  I only load a few dummy rounds in the gun so I never had that prob at home.