Check the plastic anchors that fit in the frame of the car, they could be damaged to the point that they won't hold hardware. Replace all of them and the screws. Also check to make sure the heads of the screws are not pulling through the slots in the undertray, some fender washers from Home Depot would solve this problem. You might also try some thread locker to keep the screws in place. The SVT undertray is dead easy to remove and install, $100 is about $90 too much.
