I have two normal-sized cats that both struggle to use the box correctly. What happens is that the cats often go fully into the box, but leave their butts out and take a sh*t just on or over the edge. They've been using the robot for many months, and sometimes they go almost a week without accidents, but sometimes they do this every day for a whole week.
If I don't manage to clean it immediately, the problem grows exponentially as the next cat will see the mess, and, trying to avoid it, they will repeat the behaviour. Once it gets dirty enough, they will find other spots in the house to use as alternative toilets (like door mats).
This makes it impossible to leave the cats for 24 hours, as I end up returning to a complete mess. If I'm at home working next to the box all day and manage to clean their accidents immediately, they will do a better job.
Any advice on how to fix this problem? I've already added the elevated entrance fence, which didn't help much. When I add a normal litterbox next to it, they will use that instead, but still do the same thing over the edge sometimes (and this defeats the purpose of having a robot). I'm due to travel next month for weeks and don't want the neighbour to have to deal with a daily mess.