Losing feathers can be part of the natural process of feather rejuvenation called moulting. Feathers can become damaged over time and need to be replaced.
It's possible that your other chickens are picking on this bird. Sometimes when chickens get bored, they pick on each other. Stress can also cause a chicken to lose feathers.
It sounds like you have a feather picking issue. Some strains of chickens are cannibalistic. They may be crowded, try giving them free range. High-fiber seeds, rolled oats and alphalfa hay may help.
