Just an idea - I tried every litter sold in all the major pet stores, and could not get my cat to use the litter box. Then a new vet told me to get Pestell, which is unscented http://www.pestell.com/pet/cat_litter/clumping/ .
It may be hard to get; contact the company and see who sells it in your area.
But after I tried the Pestell, no more problems! I’ve been buying that litter for years now, and all three of my cats have been using the litter box faithfully ever since!