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How many other chickens do you have?
Do the other chickens pluck feathers?
Where is she in the pecking order? (Observe the chickens over time eating, drinking, roosting, foraging, nesting, dirt bathing over time to find out. More information on the pecking order here.)
Does the chicken keep frequently going to the nester without laying?
I recommend, especially in hot weather, that you clean up that poop all over her vent area immediately to prevent flystrike (where flies lay eggs on your chicken's butt and the maggots hatch..its not good).
I don't think your chicken is sick, but it would be good to do some more research on chicken illnesses and such.