Have you ever cleaned a spot on your carpet just to have it reappear the next day? This is caused by what we call carpet wicking. Have you seen a spot return multiple times?
Carpet wicking is when a liquid fully soaks into the pile, is cleaned and dries slowly. When carpet is cleaned more often than not only the top portion of the spot is removed. Often the spills contents remains at the bottom of the pile or even the backing.
When the carpet is cleaned it is re-wetted and must dry again. Carpet only dries at the top of the pile as air moves the water molecules away. When this happens the water on the bottom moves to the top carrying the original spill with it. It leaves the coke or cool-aid or whatever on the top of the carpet making it look like the spot just reappeared.
To prevent wicking the key is to dry the carpet very fast. If a spot reappears or you simply clean it again (possibly using less water) and put a fan on the spot to dry it faster.