It has been awhile since I played ZT1, but back in the days when we were all playing it, there were lots of discussions about lazy keepers. And I thought we had determined that you had to let keepers "reset". If you kept them entirely trapped in an exhibit, they eventually stop working, or at least do a very poor job.
Keepers "reset" by coming out and then going back into an exhibit. To keep them from wandering away, I would create enclosed areas next to an exhibit that would allow the keeper to come out but not wander away. For example, I might create two or three exhibits in a row, and have one keeper assigned to care for them. Behind the exhibits I would create an enclosed alley that would allow the keeper to come out of one exhibit and enter into another. Going to and from exhibits seemed to work well.
Sounds like you have 2 or 3 keepers per exhibit that are not working, you then hire a new keeper and place him in the exhibit and the new keeper will start feeding and cleaning poo. That is because he is brand new to the exhibit, but because he also can not "reset", he eventually stops working as well.
That's my thoughts on it anyway.