You need liquid B complex not just B12 she needs this becasue the B complex [if it is a niacin deficiency ] is the best for getting them over it
https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/
Hard to say what may be going on, but we all know Pekins at any age can come down with a niacin deficiency so I would start there with the Durvet liquid B complex 1 ml over a small amount of treat she likes. Keep her from overdoing and right now walking as far as she was is over doing. I would...