oh boy, Im not a vet but just putting some ideas out there, CALCIUM, now do they have a supply of that? if they don't eat enough they can get lame as the calcium for laying eggs and raising babies is depleted and then it is taken from their own bodies and if the don't eat enough grit oyster shell and minerals they get low. they need vit D3 as well with the calcium or a source of sunlight to metabolize the calcium for use....now that said, can you take them to a vet to have them checked,? if not all you can do is guess and do a broad treatment. or take a fecal to have it checked for worms and bacteria count. be sure to look them over good, inside the mouth and check their droppings, someothers may have some more advice for you so keep checking back.