Can you feel a lump in her crop, indicating that food is getting stuck there? If so, I have had good results from feeding a diet of fishing maggots for a few days. I know it sounds gruesome but it seems to work. Most fishing shops will sell you live fishing maggots (by the pint for some reason). Chickens love to eat them and swallow most of them whole. Maggots then eat through the food stuck in the crop, thus clearing the obstruction.
Keep the chicken away from grass (long grass is often the cause of the blockage) and feed a low fibre diet.