Place a crate or cage/pen inside the coop or run. Ever day, place her in the crate for 10 minurptes with her food and water, especially first thing in the morning and in evening before roosting. Or you can leave her in there for a week or so to get her built up and strong. Some vitamins added to the water would help, and a bit of plain yogurt to her feed. This crate should be near the other chickens where she can see them.