Ben-
Check out
antennaweb.org. It's a great website that, based on your address and some key demographics about your location (location of TV in house, large buildings nearby, etc), can tell you what kind of antenna you need and where to point it.
My guess would be that since 15.1 and 2 are in a different direction than the other channels in the Madison area your antenna doesn't pick the 15's up. You might be able to adjust your antenna slightly to get both at the same time (antennaweb will tell you). If not, you may have to adjust your antenna every time.
Hope this helps!
Dan S