Remember when I told you cats have more scent receptors in their nose than dogs? If you can smell the litter box, imagine how intense the odor is for them! Litter should be scooped at least daily - yes, even clumpable litters. I scoop my boxes 2 times per day.
Given the findings about microscopic feces adhering to the granules of culpable litters, they should be dumped and replaced considerably more often than is discussed on their labels. I completely replace my litter weekly.
Wash your letterboxes every time you change the litter. Use very hot water and a mild detergent and rinse well; do not use a disinfectant like bleach; strong, chemical smells are disliked by cats, and given their keen sense of smell, even a small odor will smell strong and potentially deter them from using the box.