The other thread:
https://www.backyardchickens.com/threads/need-help.1650713/
I'd definitely just switch to an all flock for higher protein, cut the corn out totally except for occasional treats, and have a separate feeder/dish for oyster shell as the all flock has more protein, less calcium.
All flock can be fed to all aged chickens, including chicks.
To get her to eat, you could try wetting some of the feed in a small dish. Ours act like that's a treat. That would provide a little hydration too.
Scrambled eggs is something else you could try a little of as most chickens can't turn those down.