Reasons why a cat suddenly stops pooping in the litter box:
1. The litter box is too dirty. Cats are very clean animals. If a cat suddenly stops pooping in the litter box, it's usually because the litter box hasn't been cleaned for a long time and is too dirty. There's a lot of feces in it, and it smells terrible, so the cat won't go there.
2. Cats don't like cat litter. If the owner suddenly changes the cat litter, the cat may not be used to it. Some cat litters don't feel good underfoot or have a pungent smell, which will cause the cat to dislike the litter and refuse to defecate in it.
3. The cat is getting revenge on its owner. When the owner does something that makes the cat angry, the cat will deliberately avoid using the litter box in order to get back at the owner and get their attention.
4. The cat is sick. When a cat has a gastrointestinal disease, it may not have time to go to the litter box to relieve itself, or the cat may be extremely uncomfortable and no longer have the strength to go to the litter box. 
5. The cat is frightened. When a cat is scared, it will hide in a corner and refuse to go out, and will find a secluded place nearby to relieve itself. Or, if the cat is severely frightened, it may lose control of its bowels.
6. If the owner has recently changed the cat litter, for example, from bentonite to green tea-flavored tofu cat litter, the cat may not feel the soil when digging and may refuse to use the litter box.
7. Male cats may not defecate in the litter box during their heat cycle. Generally, unneutered male cats will ignore the litter box during their heat cycle and leave excrement in various corners or conspicuous places in the house.
8. The litter box is placed in a location the cat doesn't like, or it's not hidden enough. If you have many rooms and a lot of space, it's recommended that you provide several litter boxes. If you don't have that many rooms, try to avoid placing the litter box in a difficult location.
9. Excessive psychological stress in cats can cause this. If a cat is in a state of constant fear, it may not use the litter box normally. In severe cases, the stress response may even lead to incontinence.
10. It could be a urinary tract infection, arthritis, or other illness. If a cat suffers from urinary system diseases, kidney diseases, or reproductive system diseases, it will often not defecate in the litter box.
