Concentration is probably it. There' s been a lot of days when I shot better at 40 than I did at 20, just because I concentrate more on the longer yardages.
Even though the dot is the same size, it looks smaller at 20 yards than it does at 15. The smaller size focuses your concentration more. Get a package of those small orange sticky dots and put one in the center of your bullseye. That should get you settled in at 15. The smaller you aim the smaller you miss (hopefully

).