Our kitten was successfully potty-trained by week 8. He is 5 months old now and we always have 2 litter boxes going for him, I change both of them, completely, daily.
I noticed about a month ago he would get really fussy about using a litter box that he felt was full but did not seem full to me. This would happen at about 4 hrs before it was due for a change. During this time I would have to strictly keep an eye on him because he would try and go in corners, or sometimes super odd places like on top of my laptop bag.
About 1.5 weeks ago he was down with diarrhea and that's when it hit the fan, literally. It would come on so sudden and explosive, he would have little time to find anywhere suitable to go. The vet gave us antibiotics and it took us 2 days to realise they weren't working, because the problem was worms. Four hours after the worm tablets and he was perky bright and happy... 24hrs later and his poop had become a healthier colour, but still soft. Three days later his diarrhea was still hanging around, although not explosive, if he went in the litter box once, he would not want to use it again, which meant if he had his way, I would be changing it 5 times a day. I took him off the antibiotics completely 2 days ago and his poop has started firming up, but is nowhere near solid and healthy.
About 3 days ago we started letting him have free roam of the garden whenever he wanted,(after several weeks of supervising him in it) in an effort for him to be able to use that as his toilet. I've never actually seen him do it. At the same time, our neighbour (we're in an apartment building with a yard), who owns his mother, has let him visit quite a bit, once for a full day when I was at work, and she has said not once has he tried to go to the toilet in the house.
He is still pooping and peeing OUTSIDE of the litter box in our house even with his stool situation not being so flatulent! It's really stressing me out and causing tension in the household. Not only am I changing 2 litter boxes sometimes twice a day (it was 3 boxes when he was sick) but I can't relax when he's in the house because I'm trying to stop him before he can go on anything. Is the reason he does not toilet in our neighbour's house, but does in his own home, because his home smells of toilet from when he was sick and exploding everywhere?
Our neighbour's house, I've noticed, is immaculate, while ours is always on the messy side, as we work full time. Our neighbour is home all day.
Another thing that I've noticed that is ironic, is that if I can tell he needs to go toilet, I place him on the litter box and he feels it is too full and I'll have to find him a fresher solution - but by the same token, if I empty out that litter box and he finds the garbage bag with the soiled sand in it - he will immediately go toilet on that sand or on top of the bag.