It has been pointed out that I have been remiss in my duties to make this forum informative as well as fun so I am trying to do both.
With 10 new kittens in the house it seems like a good time to go over some litterbox issues. This area seems to be the sore spot that gets cats put into shelters almost as fast as scratching furniture (we'll handle that later)
Cats are normally quite clean but they have certain demands when it comes to "going". Most people are familiar with the 1 box per cat plus 1 rule but most people have no idea what cats look for in those boxes.
Cats will not use a litter box they have a negative association with.
If you have a more shy cat they will want their box in a private, out-of-the-way. place but may not like a covered box as they will feel trapped. This means they won't use it if it's wedged behind the clothesdryer either.
Large cats (like Maine coons) prefer large boxes but smaller cats can be intimidated by a box that is too large or deep.
Older cats may stop using the box if it is hard for them to get in and out of or if they are starting to develop arthritis and it hurts to get in or out. Boxes with a low or cut away front usually solve that problem as well as getting them a glucosamine suppliment of some type.
All cats are fussy about litter texture. Some of the new products are geared to human convenience not the one using the box. As a rule cats do not like to use the pellets or the large "rock-like" crystals found in worlds best litter (I tried this and had limited success. not only are the solids almost impossible to scoop but walking on any crystals on the floor is like walking on Lego blocks in bare feet. Needless to say we only tried it once!)
Which box and what litter/placement works can be a bit of trial and error but we use !/2 inch of "Fresh Results" or a similar "sawdust" type material (Arm and Hammer and Tidy cats make similar products) with a good 4-6 inches or Tidy Cats clumping litter (24/7 or Instant results because they come in 40 LB buckets and we have a large population. The empty buckets also snap shut so waste litter odors are contained)
I usually recommend people stay away from non'clumping or clay litter. It's cheaper true but it also has very large pieces and with no way to get pee clumps out the Ammonia smell of the urine can be overpowering in as little as 2 days so you will spend a lot more time emptying and cleaning the boxes.
We usually change litter every 7-9 days and we scoop twice a day. At $12 per container that means about $100 a month for 23 cats.
Although it is rare some cats will refuse to use a litter with a strong perfume smell better to add some carbon, sawdust or baking soda than stir in an Airwick solid.
If your cat stops using the box all of a sudden it can be due to a sudden change in their environment (sometimes as little as a visiting relative or change of laundry soap)
There can however be a more serious cause.
Female cats are known to go next to the box if they have a urinary tract infection. While this is uncomfortable for females it is deadly for male cats!
If you see your boy going in and out of the box a lot but not really peeing, if he strains to go or cries, get him checked out immediately. A sure sign of impending doom is where he goes to sit down and ends up sitting on one flank (side-saddle)
Don't take any chances here as the crystals blocking their narrow uretheraes will cause massive infections and burst bladders. It can kill your boy in 48 hrs if left untreated, and will at best result in massive vet bills and support care.
If no obvious causes are present a product called Litter Attract or Cat Attract litter can be added to a mild smelling litter to lure cats to the box. These products also contain a great informational booklet for troubleshooting litterbox issues.
If you have added a new cat be vigilant to "box Guarding". Similar to resource guarding in dogs the existing cat may be blocking the newcomer from using the fascilities. Sometimes putting the other box in a different location will solve any longterm problems but the cats usually sort this out by themselves after a time.
I hope this helps and feel free to use the "comments" to ask questions. That is one reason the forum exists. Please feel free to post questions at any time or use contact page to get private help.