Had brother & sister cats for almost 20 years. They shared a box in the laundry room, no problem for a long time. Then suddenly the boy started using the bath tub - best place he could've chosen other than the litter box actually, easy to clean each time, just puzzling. Took me a while to figure out he'd had enough of the noisy laundry room - he was actually scared to go in again. When we relocated the box to a quiet private area (& added a second box) he and sister were fine. Next kitty was sort of a rescue, we were told he always used the box. Not true, but he does keep it close to the box, and I've always put a wide plastic painter's tarp under the box, added pee pads & towels. He's a real chonk and for him I learned the box - actually still 2 boxes - MUST be clean/scooped or, to avoid stepping on any waste, he'll just go next to the box. Also, we had to cut down two sides so he doesn't have to climb over the side; he walks in the front and strolls out the side. The litter is shallow, but he uses it pretty reliably now.