Automatic Litterbox Central

I used by purrforma box for my old cat. When she passed, I put it away as I had kittens too light to use the box. I have recently pulled it out of the attic and put it up. The cats are now adults (18 lbs and 9 lbs respectively). When I plug the box in, it rakes perfectly. But it does not rake after the cats have used the box. Any suggestions on what might be wrong?