Are you sure he is not preening himself? If he has increased the itching recently, sometimes birds scratch and even pull their feathers out because of boredom. If your vet says your bird is not sick and has no problem, then he's probably bored and finds nothing to do but that. It may help if you buy some new and interesting toys for him/her to play with. Sometimes that's all a scratching bird needs. She may want a mate if she's an only bird, so if you think that's what she wants, you may need to get another bird. I have two cockatiels for 3 years now, and I've never had an itching problem. Have you changed her environment, surroundings, or anything in the house lately? Sometimes this causes birds to get frustrated and pluck. She may also have a diet deficiency..................what do you feed her? Sometimes birds just develop this bad habit, but its important to make them stop before they make themselves bald. Disease also causes feather plucking, so make sure she is not ill in any way.............I know your vet said she isn't, but ask him/her for suggestions on what you can do to stop her. Do you bathe your bird? If not, maybe she needs a shower!

Here's a site that may help:
http://cockatiel.org/petquestions/index.html
Its about cockatiels but I think it applies to all birds. Click on feathers to get the information you need and please keep us updated on your little feathered friend!