As already advised. Take her to a good Avian Vet and have her checked with blood work to rule out any physical reason for this behavior.
Most plucking I have seen is do to stress which can be brought on by a number of things. Loss of an owner, separation from the owner, other pets attacking the birds cage, temperature/climate wrong ie too cold or hot where you are keeping her, boredom, lack of mental stimulation, lack of physical activity.
After your vet check can you tell us how you inteact with her and her environment on a routine basis?
Some birds like my Cockatoo require a lot of physical interaction while others do not. Each is an individual so you must deal with each one based upon that birds needs.