I don't think I have ever been able to wear mules--flat or high-heeled. The closest I would come to being able to wear an open-back shoe would be thong sandals, and I think this is because of the snug grip between the toes.
I asked my very stylish aunt why this might be. "Am I doing something wrong?" I said. "I have never been able to wear them, either," she said, "and neither has Hope," (my cousin/her daughter). "But Hannah (her other daughter) has a pair of baby blue heeled mules that she loves, and she wore them to a wedding and danced all night." I can't even imagine!
What would make some people able to wear mules, and others, no? Is there some kind of foot architecture I am unaware of??? Thanks.