There are several possible explanations as to why one of your cats doesn't
use the litterbox occasionally.  First, have the cat checked with your vet
to rule out any medical problems.  If your cat is aging, she may find it
harder and harder to make it to the litterbox on time.  If your cat is
healthy, there could be many simple issues.  It could be your cat is marking
her territory.  It could also be that your other cat is interfering with
your problem cat's litterbox usage.  You may need to get a new litterbox and
place it closer to where your messy cat hangs out.  It could also be that
your messy cat doesn't like the litter, or even the cleanliness of the
litterbox.