Tim, if the soap and water doesn't work, let us know what kind of floors you have. One possibility is that if you have oiled floors or floors that have accumulated oil from "cleaning" products, this might become a magnet for dust and dirt. If so, a mild solvent like mineral spirits/paint thinner should remove it. Be very with this or similar flammable solvents and follow all the label directions. Handle any rags that you use for this like you would the solvent until you have left them outside to dry completely for a few days and never leave them wadded up.