Urine can be very tough. Are you an actuall carpet cleaner?
There are several reasons why you will probably not get the stains out after they have sat for awhile. I won't bore you with all of the particulars, but one is the fact that urine begins to break down by producing bacteria and enzymes. During this process, one of the off gasses is ammonia, which will set the "stain" permanently. However, depending on the type of carpet (olefin, nylon, poly) you may be able to lighten the stain using a peroxide based urine stain remover, such as prochem's urine rescue. If the homeowner has tried everything on it, forget about it and move on. You do not want to take responsibility for the damage they have caused that untill you cleaned it was non apparent.