This sounds to me like an environmental problem.

I would re-examine everything from top to bottom.

1. where are you keeping the rats?
2. is there anything in the room that could be killing them?
3. are you using any chemicals to clean the enclosure?
4. what is the temperature in the room?
5. what are you feeding the rats?
6. could there be anything in their water?

You need to look at everything from start to finish. Rats are notoriously hardy creatures that should not be dying off like flies. Their needs are relatively small and they survive in some very tough conditions.