The rear case holds two small plastic covers for the adaptive trigger assembly adjustment sliders. These may have come off during removal. Place them back in the rear cover now to avoid losing them.

Make sure it is placed so it can move freely in its groove. The edge with the notch should be towards the middle of the controller.

If a cover got bent during the rear case removal or reassembly process, apply a small amount of heat with a hair dryer to restore it to its original shape.