Monistat is given twice a day for 7 days as described in the article.
https://www.backyardchickens.com/articles/how-to-treat-sour-crop-and-impacted-crop-and-how-to-know-which-one-youre-dealing-with.73607/
If you have a way to cage her within the run or coop so she can be near the flock, this may...
Please post some photos of your hen and her poop.
Sounds like she's still molting if there's still keratin sheaths covering the feathers.
Birds that are molting do not feel well and can lose a bit of weight of not feel as robust since they have a lot of feather loss.
It's very hard to gauge...