It sounds to me like either the sear is not being fully retracted when the trigger is back or that there is a burr on the bottom edge of the striker where it engages the sear. Are the trigger and sear stock? Did this start after you or someone else tried tuning the trigger bar?

Pull the slides from your Shield and another of your M&Ps and look at the sears when the trigger is pulled. Does the Shield sear retract the same as the other M&P?