You need to pick her up, and take a really good look at her--are the feathers broken off, are they just falling out, or being pulled out (any signs of blood or pecking?). Is she completely clear of lice or mites? Do you have a rooster? I would also sit and watch the flock, pull up a chair and watch them for at least half an hour--is she being harassed? chased away from food? over-mated? what do her droppings look like?