Now that you have her warmed up, it would be a good next step to perform a full physical examination and check over her body for any abnormalities that may help further narrow down her problem.
A lot of the time it's easier and more efficient to check head moving to toe. Check her eyes, inside her mouth with a flashlight for any lesions, tackiness, plaques, mucous, etc - pull each wing out and see if she retracts both quickly with little effort. Feel her keel bone, and use the scoring graph below as a guide (a Pekin should be around a four). Palpate her abdominal area, and note whether it feels distended, and if so, if it feels hard, soft, or like liquid. Check around her vent, and look over her integument for any sign of ectoparasites. Fill free to use a similar sized duck as a guide to what "normal" looks like.
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https://www.merckvetmanual.com/exot...ltry/physical-examination-of-backyard-poultry
Is the duck eating, or drinking much at all? Do you have any pictures of her droppings, if so, could you post them?
I would try to get a weight on this duck using a large gram scale. If you have some electrolytes, such as Sav-A-Chick, try warming those up a little bit, and offering them to her. Some warm mashed scrambled eggs or warm soupy feed may be enticing for her as well.