plucking--help

I have a yr and 1/2 old female grey who is very friendly, cuddly and chatty. She is the sweetest thing!! I have had her since she was 4 months old. She loves me to death and is getting used to my husband. Her routines have not been changed and things are the same here,etc. About 2 months ago she started picking and plucking her wings. I took her to a vet who was recommended as an Avian vet and found out she wasn't. She gave me 2 meds and said she thought she had an infection. I gave her the meds ,but she was still picking and moving to new areas of her body (breast and neck now). So I found a REAL avian vet and he said he would not do tests as they would come back negative since she had been on meds. He recommended a diet of new pellets (Harrison's) for her as she seemed deficient . She hated pellets and would just trash them until I started these new ones. They are only sold online or at vet offices. She actually loves them!! I am thrilled she finally is eating pellets!! I have tried many diff ones and no luck !! He was hoping she would stop plucking with a better diet. I have a follow-up appt next Monday and he said he will do tests then if necessary. He warned me it can get very expensive. I am very concerned. I have read it can be psychological and I hope that isn't it. I see no reason she would start this out of the blue??? It is very confusing and upsetting to me!! My beautiful baby is now looking very shaggy and homely. I feel terrible and wish there was something I could do to get her to stop.
Has anyone had this problem and what did they do?
What did the vet say and what tests did he take if any?
Anyt thoughts or help would be great!
Thanks
Sher