Returning spots are most commonly caused by deep down dirt.It is usually dirt or a spill found at the carper backing (this is the trap -like material behind the fiber) or in the cushion padding below the carpet.Once these spots have been re-wet by cleaning they tend not to show up right away but do once the reying process begin.This is what the professional carper cleaners cal "wick-back".If you are not able to hire a professional carpet cleaner to manage with this then the best way is to not over wet the carpet and re-active the deep stain.Or you can clea it using your method and once it dries and the spot re-appears use a lightly damp towel and only work the stain out of the top fiber.Do not over wet.Wipe gently as not to distort the fiber.The spot will stay there yet it will be below the surface and will not be noticeable.You can also follow this advice:to avoid the return stains covet the spot area with a tick towel (after cleaning) and weigh the towel down with something heavy:a brick,book or other object.Leave the towel on the spot overnight to absorb the liquid and clean the remaining stain.Have in mind that a good professional cleaner is able to properly remove it.