If a healthy cat is not using the box, try moving it to where the cat is eliminating, in case the cat has a location preference. Usually however, the only preference is that the cat would rather use the clean rug rather than dirty or smelly litter box. Litters You may also want to try different litters, but avoid perfumed or scented litter. It may smell good to you, but lots of cats can't stand it. Consider trying a clumping litter, if you don't already use one.
Many cats prefer the texture.