The cat in question is a 5 year old female who has been spayed. We keep 3 litter boxes, each of varying sizes, around the house in separate, low-traffic areas of the house, but all are very easy access to the cats. They are also cleaned regularly. We have four cats in total, all female, but only one of them does not use the box. Instead, she goes right beside it. There does not appear to be anything medically wrong with her, and she goes on the floor even immediately after I have cleaned the boxes. Any advice would be nice.