ya know, my question exactly...i have the exact same problem...3 good miters, 1 crappy miter, and it doesn't matter what i do, i can work on the binding and get around 2 then stop and go back later to do the other 2...somewhere along the way, 1 miter does not ... miter...it's a crazy thing; helps to keep me humble i guess; if i ever get 4 perfect miters...LOOK OUT WORLD!
so i have no advice how to fix it, been trying for years...