What about reoccuring spots?
If the spot has returned within a day or so after the cleaning, then it was either not removed and wicked back, or it was not removed but appeared that it was removed. If the spot returned after a week, two weeks, or a month or so, it probably either has some resudue from cleaning agents left behind, or the spot was oily or sticky and was not totally removed therefore collecting soil in that same area again.
Note-DO NOT USE OVER THE COUNTER CARPET CLEANERS OR HOUSEHOLD PRODUCTS BECAUSE THEY ARE TOO HIGH IN ALKALINITY AND LEAVE A TREMENDOUS AMOUNT OF RESIDUE.