Are they plastic?? Every few weeks, we empty the litter, clean the box really well (soap/water, etc), let it dry or dry w/paper towel, put fresh litter in it & it's good to go til the next time. I've honestly NEVER had litter box odor. Only w/this new cat have we had to clean it more b/c she seems to 'go' more. Maybe it's the litter you're using. Have you tried a new brand or one that's more for odor control or perhaps sprinkling baking soda on top? Just keep in mind, cats may have issues w/not liking 'new' litter if it's a different scent or different brand so if you do try a new litter, try to find a new scent, one for odor control or unscented in the same brand if you can't, or even if it's the same brand but different scent, just be sure to mix it a little at a time w/your old litter so the cat will get used to it. Cats are just funny that way but they may avoid the litter box if you just all of a sudden start using new litter w/o blending it in to transition to a new litter. Also, be sure to check around the room to see if the cats may've urinated on something like the wall, etc. Sometimes cats will urinate on stuff they're not supposed to so you may have to check the room & clean that up if you happen to find some. Hope this helps, good luck.