Did you happen to change the shelf material at the same time? Going to a softer shelf material will sometimes drop the point of impact. I use a Martin rug rest, and as it wears, my point of impact rises; when I replace it, my point of impact falls.
Or if you have a gap between your shelf material and the strike plate, is it possible that the arrow is settling too much into the gap?
The thing that strikes me as odd, and I have no explanation for, is why lowering your nock point wouldn't raise your point of impact.