The M&P guns do not have a disconnector. What is under the rear sight is the striker block. This could cause issues, but not what the OP is experiencing.

Groundhog34,
The very first place I would look is the trigger return spring. If the trigger is not moving forward, it could be this spring. Of course, if it were, I would also think that S&W would have noticed. However, their claim that it was the sights is ludicrous.

Do you have any pictures of the gun? Especially with the slide removed? I would like to see the bottom of the slide and the inside of the frame.

Here's a little video I made on how the trigger works in an M&P:

EDIT: The Shield 45 trigger is a little different at the sear. That part isn't included in my video. Most of it is the same though.

__________________
Freedom isn't free.