Stain Wicking

Stain Wicking is a very common occurrence with carpet.
 When carpet is cleaned the fibers are cleaned in a consistent way.  If there are spots that went deeper than the water and suction combination can extract, then they may return.
The water becomes the way in which the stain wicks. The wicking is from the dirt being too deep to lift.  It matters little whom is doing the cleaning,  it' more about the efficiency of the equipment and the operator recognizig the spot.