This is usually caused by a chemical residue being left in the carpet. This residue can attract and hold dirt. Our process leaves virtually no residue. Steam cleaning or hot water extraction (same thing) relies mostly on water temperature rather than chemicals to clean. If you took a greasy pan and ran cold water over it, the grease would barely move. Now, use very hot water and see the difference. Very little cleaning product is necessary. This is one of the reasons steam cleaning is so effective.