There are two reasons that spots return:
Reason # 1: Wicking. Many times, the spot that is being removed 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” of the carpet is removed. However, moisture is introduced to into the backing of the carpet. As the spot dries, the material that is in the backing of the carpet “wicks” back into the pile of the carpet much like a candle wick.
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 yourself, 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. Be sure to use something non porous that won’t transfer color onto the carpet (like a paper book cover). Use a brick, a planting pot, or something like that.
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.
Reason # 2: 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 cleaners all together. If you have to use one of these cleaners, or you have to use dish washing detergent, use as little as possible and rinse with water thoroughly.
Other residue causes would be oily spots. Duct tape residue, body oils, lotions, etc. If all of the oily residue is not removed, it will attract soil as the carpet is walked on. Even quality professional cleaning may not remove all oily residue the first time.
SPECIAL NOTE: Cheap carpet cleaners use cheap products that are loaded with Optical Brightners. Optical Brightners create an illusion of clean that causes the soil or spot to appear to be removed. The spot is still there. The soil is still there. Optical Brightners create a film that reflects light so that the carpet looks incredibly clean. It’s an optical illusion.
A stark illustration of this can be seen at a flea market or home show. Someone will be demonstrating a miracle cleaner on a small rug. The rug is saturated with motor oil. The salesman then scoops off just the foam of the miracle cleaner and presto, the motor oil is gone! This is physically impossible. Notice that the salesman has several bright lights shining on the demo rug to create the illusion. - Towers Restoration and Cleaning Carpet & Floor Care - Midlothian Texas