Although most stains that are removed from carpets do not reappear, from time to time we run into situations where spots have reappeared after the cleaning. There are 2 reasons why this could happen. 1) Something in the pad or the floorboard (usually pet urine) has worked its way back up into the carpet. This is more common than you might think. The pad works like a huge sponge absorbing anything that penetrates it. There is a property of physics that allows the salts in urine to seek airflow, which means the urine finds its way back into the carpet even when the substance is seemingly dry. 2) The second reason spots reappear has to do with PH levels. Sometimes the PH balance gets shifted to one side or the other of the pH scale. This could be because spot cleaning efforts previously upset the pH balance before we arrived, or rarely, our agent reacted with some invisible thing already there. If your situation is a PH issue, we are happy to make a no-charge visit to correct the problem, which usually involves spraying a neutralizer on the spot and brushing with a carpet rake.