Good question, but most renditions of the story describe some tar or fly-paper / adhesive which forced Cinderella to 'ditch' the shoe and run away bare-foot. But even if we pretend that there wasn't an adhesive on the steps- considering the fact that according to some renditions of the story Cinderella is also undergoing serious change in her bone-structure (as the affects of the spell also changes her physical appearance on top of her clothing)- it's entirely possible that there could be a shift in bone structure in her foot which would cause the slipper to come loose. However, this is unlikely since I'd assume that both her clothing and her appearance would degrade at the same rate. But who knows? It's Magic!
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