The first reason is wicking. Many times the spot has seeped into the backing of the carpet. This is common with pet urine, beverages, and anything that is spilled in “volume”. The material dries in the backing and the cushion (pad) of the carpet. When the spot is cleaned, only the material in the “pile” (visible fibers) of the carpet is removed. However, moisture is introduced into the backing of the carpet. As the spot dries the material in the back of the carpet wicks back into the pile.
The second reason spots return is re-soiling. Re-soiling is caused by a sticky residue that has been left behind. The residue may be from the spot itself, the cleaner that was used or both. If the entire residue is not removed, it will attract soil as the carpet is walked on causing the spot to “reappear”.