How is your hen going? Ive had a similar thing healthy birds losing weight and losing strength and balance, in both roosters and hens, a few years ago now, one every now and then. They were all rescue cases from different places (like the roadside), had plenty feed, no paracites and were free range, shut in at night. So were warm and not stressed at all, so no obvious reason for weight loss. The vet said the most likey cause was maricks (sp?) disease. Do you have this in Eng? I think the most obvious symptom is the eventual paralysis or weakness of the legs. They stopped eating, is your hen eating? Good luck for her.