The answer to this is very simple......needs coarser grit.
Since @Juliet Balfour said that she was using 80 grit possibly there is something else going on ... perhaps a crack in the wood that actually isn't a scratch. This is just a hunch since she mentioned a single scratch rather than "scratches". Otherwise, I agree with what you say. If the scratch is really deep or if the wood is really hard then it might be necessary to use 50 or 60 grit sandpaper. Then there's the adage about using sandpaper like somebody else is paying for it.