Clodhopper, from what you say I´d give her a good dose of niacin. She doesn´t sound sick as such. If she were egg-bound I´m sure she´d be quite sick by now...temperature, etc.Oh forgot, one more egg in the nest. Might have three males and four females.
Does she like a flock mix or chick crumb? Maybe give her a good measure of this with a good dose of niacin (not masses, of course!) and see if she picks up then. Just because the others are ok doesn´t mean that it´s not a niacin problem. Some suffer from it, some don´t.