Feather picking/destruction can be/is caused by everything/anything. There are a lot of theories out there, but what works for one parrot won't work for the next five.
If you want to try to work on the problem, you first need to see an avian vet. Your bird, like all other companion animals should be seen by a vet every year for a health exam. If you go for a feather picking profile, you're looking at checking the blood which will tell you about infection, anemia, hydration. A gram stain will check the feces for yeast, bacteria. Oral and vent cultures will tell you about some common bird bacterial infections. A chemistry (chem 12) will tell you about organ function, uric acid levels, calcium, muscle. An Xray to look for things like possible metal ingestion, and liver size. And, depending on where you got your bird, and if you've never had one done, a psittacosis test. Where I work all that's around $250 (without the psittacosis test) and that's cheap.
After all that, you have to decide on a course of action. You can do doxycycline injections, hormone injections, supplements, bathing, none of which may not stop your bird from plucking.
I do recommend that everyone have their birds medically checked every year, and that would be the blood, poop, oral culture, and an Xray depending on the age of the bird. If there's a problem that shows up after these tests, you can proceed further. If you need an avian vet, try here: http://www.aav.org/lookup
You can try bathing it more often.
It must have toys, lots, and try rotating them.
It MUST have a minimum of ten hours each night of dark, undisturbed sleep. Most parrots live close to the Equator where the amounts of night and day are equal (twelve hours each), so that's the amount of sleep we need to give them in captivity, as well. Sleep deprivation can cause irritability, nervousness (sometimes associated with feather destruction) and illness.
Feeding parrots well is not giving them fruit and bread sometimes. They need a balanced diet just like we do. Depending on the species to be more specific, seeds should never make up more than 40% of the diet. Even my small lovebirds only get about 30% seed in their diet. They get, along with the big birds, sprouts, cooked beans, fresh fruits and vegetables (more veggies than fruit), nuts, and pellets.
I have two birds that came to me as pluckers. I know it's frustrating, and you wonder what you might be doing wrong, or not doing at all, when you can't get it under control. Over the years, I've learned that if a bird does a little plucking, and no harmful self-mutilation is happening, it's best not to stress over it too much. Parrots do pick up on those kinds of things--they can read us VERY well--and when we're stressed, it stresses them.
The best of luck to you.