As a general rule, you should have one more litterbox than you have number of cats. If you've recently made any changes to litter, location, or the presence/lack of a hood on the litterbox, change it back. Are your cats getting along? Lastly, does anybody have skin problems or some other medical problem that may be causing discomfort. Have known of a couple of cats with skin allergies who, once they were no longer itchy, went happily back to the potty pan.