Hiya Mary

If she can smell where she has done her toilet before she will go to the same place again. There are various good products on the market for completely removing any trace of it. If she is tending to go in the same place all the time, another trick you can try (its a bit drastic) but put her food dish (or a second dish of food) on the same spot. In my experience a cat will not do its business anywhere near where it eats.

Good luck
Christine