Automatic Litterbox Central

We recently got a litter robot and have 2 cats, Hansel & Gretel. One of the cats seems to use the litter robot, the other won't. So for now we have left a small littter box next to it, which we don't empty in the hopes she will switch to the litter robot as well.

However, the one that uses the litter robot seems to keep peeing on the ledge of the entry with some going on the tray below. He seems to realize what he has done because there is always litter kicked over it.

Any suggestions on how to get him all the way inside the litter robot before he pees?

The poops are all the way inside, so unless its a question of one cat going all the way in and the other not, I don't understand it. (I've never seen either cat actually go in the thing - I'm just looking at what's in there when I run the cycle. We still have it unplugged.) And I end up having to clean the lip most days when I run the cycle.

wendy