I had a cat that NEVER used the box....I asked everyone what to do.. and I got a BUNCH of ideas..Some sound crazy... but I did it anyway! But then again, if it's behavorial, then changing the litter is going to do nothing.

I have, used top soil, news paper, sawdust, sand, clumping litter, clay litter, litter called "Attract" it's supposed to have a smell in it to make them want to go in the box, any combination of any of the above. I have changed colors of litter boxes,
*yes colors... I was told cats like blue litter boxes the best*
I had 3 litter boxes in a room no bigger than 8 x 12, and shut her in the room. Put her food where she went potty, used every cleaner on the market and any home made mixture I have ever heard of, changed cat food. All of this... for three years... and nothing ever changed. AND she was an only cat!
I did everything I could to not let the hubby know about accidents, but when she peed on my side of the bed, and I had to sleep in it so hubby didn't know and get mad at her.... I knew I could do no more.
I trapped her as a feral kitten in a neighbors barn and had no choice but to release her back to the outside... maybe that's why she never used the box. She wanted to stay "wild"
(sorry for blabbing for so long)