
There are so many causes of illness, it can be hard to find the right answer. If you have a vet that can perform a fecal float, that may prove very helpful.
If she will stress out from being separated, then leave her with the others if she isn't picked on. Stress can make things worse.
Sour crop can be caused by numerous things, moldy feed is one of them. Worms, coccidiosis, reproductive problems can impact crop function. I would check her crop to make sure it's emptying overnight, just to make sure that part of her is working.
Hopefully @Eggcessive will chime in with her thoughts.