I have a CMC flat trigger in my M&P 15 Sport II. It works perfectly!
In my M&P 15-22 I have the Hyperfire 24C. It also works perfectly!
Before I had these triggers in these two rifles I had ALG-ACT triggers which also worked perfectly, but wern't as good as the ones I have now.
I think if you are having problems, you should probably remove the triggers and re-install them with the manufacturers instructions. You probably won't have any issues if you do that!