There are 2 main reasons why spots sometimes return;
1) "Residue Inducing soiling" - This is when a sticky residue remains in the carpet after it has been cleaned. For example in juice, the sugar can attach to the carpet fibres and over time the dirt will restick to these residues. Regular vacumming will not be able to remove the dirt from the fibres so the spot will eventually reappear.
2) "Wicking" This is a process where excess water can trap dirt below the carpet fibres. As the carpet dries, dirt is pulled back up along the fibre and to the surface - leaving you with the same spot you tried to remove.
This is why the Chem Dry system is superior. Using only 1/5 of the moisture most other cleaning processes use, means wicking is almost impossible to occur, and as Chem Dry doesn't leave behind sticky, soapy residues, it means spots are far less likely to reappear!