Chickens eat to fill there caloric needs, if you feed them a feed that is overly high in calories then most chickens will tend to eat less.
When you feed a proper feed/diet that also has a high amount of proteins in it you are building the muscle and also giving them the proper energy they need...
I would put her on a good poultry feed and one that has animal proteins in it, a good game bird feed would work great since it is a higher protein feed and most good ones have animal proteins in it.
The other thing I do is hit her with a good wormer, dust her and treat for cocci.
Keep her off...