Thinking outside the box;
Worms of some sort 

- what do their poops look like ? An icky photo would, possibly, help see any internal parasites.
More information 

 - How long have they been like this ?
More information 

- When you say they won’t eat their grain, are they eating the rest of the food - corn, pellets etc ?
Some weird illness 

 - Are they drinking and are you replacing the water everyday with fresh ?
Stress 

 - How much space do they have ?
Hate to hear of anything like this, of course they may be hanging on for the goodies - they can be precious little s*ds 

Hugz
One other question ? Are they laying ?