Make sure that she is getting enough food. She may be more meek and afraid to approach the feeders. Take her aside if possible and feed her some wet chicken feed, scrambled egg, or even a little canned cat food to see if she will fill her crop. Her crop should be full by roost time and mostly empty by morning. Things that can prevent weight gain are worms or a chronic case of coccidiosis. Try worming with Valbazen or SafeGuard, and feeding separately.
Symptoms of coccidiosis are lethargy, diarrhea, poor appetite, and appearing puffed up. Corid in her water for 1 week is the treatment for coccidiosis. If a vet could test her droppings locally, you could rule out worms and coccidiosis.