Sounds like your bird plucks similar to my bird (mine plucks her back underneath her wings, and her under-wings, as well as her legs). Definitely get her vet checked, my galah had organ failure and a severe calcium deficiency. Now that she's healthy she plucks less and her bald back has a few new feathers.
What is your parrots diet? Large parrots and cockatoos shouldn't get any seed in their diet, she should be eating lots of dark leafy greens (collard greens, spinach, broccoli, dandelion leaves) squashes, peppers, and much more. The occasional nut(no peanuts, use unsalted almonds, pecans, pistachios) is also fine. Cooked Grains are also a important part of their diet, my favorite is quinoa but I also feed barley.
Probably one of the most important parts of this diet is pellets, use a high quality pellet like Totally Organic pellets or Harrison's Pellets(I fed both). Pellets are the back up plan to the above foods, they fill in the gaps of missing nutrients and should be the staple diet. Feeding 50% pellets and 50% fresh/prepared foods is the best.