If you can go into your coop at night and take a bird off the coop, you can inspect for mites. The common roost mites will only come on them at night, hiding in the cracks and under roosts in daylight. The northern fowl mites stay on them day and nite.
I would check inside her vent for a stuck egg by inserting a clean finger 1-2 inches. Have her egg shells looked normal? How much Corid are you giving? I would also consider worming with Valbazen or SafeGuard Liquid Goat Wormer. Try offering some watery feed as well as the dry. Tuna is also good in small amounts.