How to fix a spot on new Poly finish?
Unfortunately it sounds like you will need to sand out the areas that are affected. If it has a coat over it, there's nothing you can do to retroactively that will fix the problem because it's effectively shielded by the top layer of finish. I think your problem might have been too course a grit on your sandpaper. I would use no courser than 280 or 320 for a poly finish because unlike a lacquer or conversion varnish (which have harsh solvents that actually melt the surface of the previous layer) poly has to float out over whatever is there. Good Luck!