Why won’t my cat use the self-cleaning litter box?

In order of likelihood: the litter changed at the same time as the box, the litter or a deodorizing insert is scented, the entry is too high or too enclosed, or the machine cycled while the cat was nearby.

Change one thing at a time, in that order, and give each change a week. Two changes at once produce a result you cannot interpret.

Start with fragrance

Scented litter and perfumed deodorizing cartridges are the most common single cause. Unscented litter and an unscented carbon filter cost the price of one bag and one pack.

Then the substrate

If you switched litter and machine together, go back to the old litter in the new box. Cats object to substrate changes more than to machinery.

Then the shape

An enclosed drum is a tunnel with a moving floor. Open-topped machines exist because a real proportion of cats will not enter one, and they are not expensive.

Then the noise

Lengthen the cleaning delay to the maximum the machine allows, and run manual cycles only when the cat is out of the room.

Keep the old tray available throughout. A cat with no acceptable option does not learn to like the machine — it finds a corner you like less.

If a cat that was reliable starts going elsewhere, mention it to your vet as well: a change in toileting is one of the things they ask about.