Hi Maisy.
Sounds like one of my ex-batts. I got them in August and three settled in fairly quickly and started doing normal chicken things, like scratching around and trashing the garden. But Mavis never did. Just sat hunched up, beak under her wing, under my daughter's slide. Does this sound like yours? (Except the slide bit, obviously!)
I kept an eye on her but didn't know what to do as there was nothing obvious wrong (apart from being bullied by the others) and she was eating and drinking. Then on the August Bank Holiday I picked her up to check her over and she was like a little chicken-shaped hot water bottle - she was burning. Especially under her wings. And she did seem all ballooned up.
Took her to the vet and she could find nothing at all wrong but gave me a course of Baytril and some Flubenvet.
Anyway, I spoke to the BHWT, where I got the girls from, and we discussed the options. I was adamant that I wasn't going to give up her as long as she wasn't suffering. The BHWT said that problems with ex-batts were often psychological, not physical, and all you can do is hope for the best. Some respond, some don't. So the decision was taken to stick with it and see how she went and to think again if she stopped eating and drinking or ent any more downhill.
I separated her from the others for part of the day (but they could still see her - I simply shut her in the run and the others out), so she could eat and drink in peace, which seemed to help.
Six weeks later, she's like a feathered football! Bigger than the others, laying again, and joining in with all the chicken things. Others are more accepting, although they do have the odd punch-up. Even dust-bathing in the flowerbeds, which makes me really happy to see.
Sorry for all the waffle above but I would stick with it, if I were you. If she doesn't appear to be suffering, and she is eating and drinking, then give her a chance. As the BHWT suggested, it may be psychological rather than physical. Keep an eye on her and give her lots of love, but make sure she eats her mash (are you giving them the ex-batt stuff?) as that's where she gets the best nutrition and, IMHO, I would keep treats to a minimum at the mo.
Be careful with bread - I think it can clog up their crops. And not sure about the cod liver oil...? Someone else can give advice on that. I wouldn't try to be too inventive - stick to good quality proprietary food, some mixed corn/mealworms and the odd bit of veg. I can only suggest this as it's worked for mine - someone more experienced might suggest differently.
Hope I've helped a bit - love to Fifi x