Perhaps, it's the bridge then. If there isn't anything restricting the spring from pushing on the bridge as it should, you might raise the bump on the bridge using a punch etc. from the backside of the bridge (question mark, graphic below). The bump has a mesa, so maybe use a flat punch. Another thing, the bridge might have acquired a dip where it should be flat. Another option is transplanting a bridge from a donor switch. The bridge is probably generic.
