Have you thought about what may be happening within the guts of your hens? If not, they may look like this:
However, if your hens have an infestation of capillaria (hair) worms, there may be little to see unless you have a microscope, but the gut lining could be even more damaged than by the roundworms (ascaridia) in the picture above. Some free range farmers seem to be complacent about...