i have 4 cats. 3 of them are a year old and the other is a kitten. the kitten uses the litter box but for some reason my older cats have not been wanting to use it. they go all over the house and it is really starting to get annoying, this has been going on for about 2 months . i keep there litter box clean and i use the same litter as i have since they were kittens, the litter i use is a all natural litter and it works great for the smell. how do i fix this problem?