Are your chickens shaking their heads about every 5-15 seconds, as if to shake off something on their necks?
If so, then it could be mites. Please note that when my chickens had the Northern Fowl mite, they were shaking their heads as I described, and I could never find one on the chickens. I found them on ME (and they bit me).
http://ohioline.osu.edu/vme-fact/0018.html
Also, they might have worms:
"Heavily infected birds may show droopiness, emaciation and diarrhea."
from
http://msucares.com/poultry/diseases/disparas.htm
There are treatments for both. Mites and worms can kill chickens.
Here are treatments for you to look up on BYC search and google if you want:
Sevin dust 5% (garden dust)
Ivomec Eprinex pour on (kills internal and external parasites- mites and worms)
Ivomec Ivermectin pour on
Ivomec Ivermectin injectable
Wazine (some say that if you think it is worms give this first or the other wormers could kill your hens)
Permethrin poultry dust
There are egg withdrawal periods for some of these meds. (Tossing eggs.)