I heard you should have 1 litter box every cat plus one. So if you have 2 cats then you should have 3 litter boxes.. so if you have 12 you should have 13. Are you cleaning the bozes with disenfectant too? Because if you are it may be a harsh smell which could keep her from wanting to use the litter box. You may want to take her to the vet just in case. She may be sick with something that you are unaware of and she might my trying to let you know. People suggested different litters because cats can be picky. Try lots of different litters and so as not to uspet your other cats don't put them in all the boxes just a few maybe she can find a litter that she likes. Good luck.