Stolte recommends putting
multiple boxes in different places in the house,
trying new types of
litter, and if all else fails, going back to square one by placing a
box in a small space where your cat was first trained.
Cats tend to view that as one large
box, and if they don't like to share a
litter box, you loose the advantage you're
trying to gain by having
multiple boxes in the first place.