Yes, I use Slic3r, maybe that makes the difference?
Hmm... the only other factor I can think of is the material you use.
I have printed a few belts and they work just fine.
I chose the tolerances as tight as I could get away with without having problems on my setup.
kourindouhime, I am planing to make the belt better but I can't work on it the next weeks. Maybe come back later.
Just making the holes bigger does not solve the problem because then the hinges are to weak.