Welcome Sunshine the Senegal, very glad you have joined PC. As you can see by reading the information Karen has provided, there are so many reasons for pet parrots to pluck and I sure wish that it was an easy thing to determine, but the truth is that you may never know. You have already taken a very necessary step and had Sunshine examined by a good vet to see if it is caused by medical issues.
There is a lot of information out on the internet about foods that birds are often allergic to, for example, corn or wheat. You would start by taking one food at a time out of Sunshines diet to determine if that particular food is the problem. This link is a good visual for the healthy parrot diet pyrimid.
Parrot Food Pyramid Diet Nutrition Healthy Bird Foods
I would recommend putting lots of clean branches in her her cage so she has lots to chew. Since she doesn't play with her toys, she may be missing something that she likes to chew and taking it out on herself. Branches would be closer to a natural environment and she may like that.
See if you can determine what in her life that causes her stress by her reading her actions. Is her cage by a window or in the middle of the room in lot's of activity?
Again, welcome to our board, looking forward to chatting.