If you change litter, put a small amount (1/4 of what you would normally use - put new, 3/4 use old), of new mixed with old. Gradually as you change the litter, increase new litter. This will keep them from not using the litter box. If you find number 2 on the floor, with something covering it, next box leave out the new. Cats have various ways to show if they do not like something. Some cats don't like a certain kind of litter, don't like the feel on their feet.
If you change it everyday and use the clay litter, do the same substitutions.
Good luck. We use the Fresh Step and they have a new one now. So far they are ok with the new one.