Automatic Litterbox Central

We recently did our second 6-month complete cleaning of our litterfrees (we have 9 of them), and after putting them back together, three of them no longer fill all the way before starting the churn cycle.

One of them always reports Er2 right as the drying cycle begins.
Another one did also, and I added some water manually as it was churning. Now it doesn't report Er2, but it still won't fill enough.

We've read this forum, and nothing sounds quite like what we have found.
We know that some of you on the forum have opened up the motor unit and done some cleaning there - is there something we should be looking for here to resolve our problem?

-Andy and D