The simple answer is that they were never removed completely. Often times, DIY methods or steam cleaners push the stains deeper down into the carpet. Once the carpet starts to dry the stain and its nasty odor is wicked back to the surface. Chem-Dry's P.U.R.T. method works differently and aims to eliminate the entire stain, not just mask it for a time. That's why we say if we can't get it out then no one can!