Could it be the type of laces you have, i.e. nylon instead of cotton? I've never heard of that happening before. My shoelaces tend to get a little tighter because of my feet swelling slightly, but I really don't know. Did you do a search on the web? You can also do a search on Spark and find a possible reason. I'd try cotton laces though to see if that could be the problem. Also, I tie a bow and then a knot to make sure they don't come untied.