Put her in a crate with a litter box and see if she uses it then. If so, leave her in there unless you are watching her for a week or so. If not, try different types of litter...clay, scoopable, equine pine pellets, pearls, etc. Also when she has an accident, put the solids in her box or a paper towel with her urine on it in the box so it picks up her scent. Meanwhile, clean up all accident thoroughly and after dry, spray them with a deterent.