Maybe she's not getting the nutrients from the ground that she needs. Have you tried other foods? I know mine love pasta, hard egg yolk and dry or runny oatmeal.
If any of mine don't eat, I tried giving them rolled oats and ease them back onto their proper chicken food.
One of my girls, Beatrice, pecks hopelessly from the ground despite fresh food being offered. One time she even got a worm!
Are there possibly any bugs she might be looking for? I know my chickens won't give up until they find the bug they've been looking for. Here's to hoping it isn't too...