I would suggest perhaps moving the litter trays to a quiet area out of the way of general "through" traffic so to speak. If they are open trays try getting a covered tray or vice versa. It may be they may prefer a different type of litter - if you have clumping - try non-clumping or recycled paper or wood pellets. Just try the new litter in one tray and see if they use it rather than the current stuff you are using.
You could try this stuff for eradicating the smell of where the cats have been
Urine-Hater. Eliminates Urine & Pet Odours - £9.99 : Pinky Pawz - Simply ... the cats whiskers!, Pinky Pawz - Simply ... the cats whiskers!