Soapy, foamy detergents have been used to clean carpets for DECADES! Your reappearing, "mystery" spots are probably from the soapy cleaners you've been using. Shake up your cleaner and take off the cap. Do you see bubble or foam? THAT is what's causing spots! OMG! Carpet Cleaning uses a non-soapy, non-foamy cleaning solution so there's not sticky residue.