The first is reason is called 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 large volume on carpets. The material dries in the backing and the pad of the carpet. When the carpet spot is cleaned, only the material in the “pile” of the carpet is removed. However, moisture is introduced into the backing of carpet. As the spot dries, the material that is in the backing wicks back into the pile.
Even traffic area soil that is embedded can wick back. This is especially common in glue down carpet that is heavily impacted with soil. When the carpet is cleaned, it looks good. When it dries, the spots and traffic areas come back.The good news is that if a spot actually wicks back, it can be removed. When spot cleaning, place a folded white terry cloth towel over the spot just after you clean it while the spot is still moist. Place a heavy object on top of the cloth.
When you have your carpet cleaned professionally, be sure to point out spots that have wicked back in the past, so they can be treated properly.
The second reason carpet 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. Some common residue causes are over-the-counter carpet cleaners purchased at the grocery store. Avoid using these carpet cleaners altogether. If you have to use one of these carpet cleaners, use as little as possible and rinse with water thoroughly.
Other residue causes include oily spots: duct tape residue, body oils, lotions, etc. If the entire oily residue is not removed, it will attract soil as the carpet is walked on. Even quality professional carpet cleaning may not be able to remove all oily residues the first time.