The reason I asked about the elevation difference is because a shallow well jet pump is limited by it. Not only is it limited by the height it can lift, but also the additional height affects the max pressure it can produce. In other words, if it doesn't have to pull as much, it can push more.
If your pump was able to exceed 40 PSI but no longer can, a drop in the water table is one possible factor. I agree with justwater, that it is probably a lousy pump, but throwing more money at a new shallow well pump cannot bribe the laws of physics so it behooves you to check it out. If it is borderline, now might be a good time to consider a submersible.