Hi,
I would agree w/the above statements. U really might want to figure out when, where and how much she is scratching. If the skin doesn't look irritated, I wouldn't go to a medicated shampoo tho. Rinsing well after a shampoo or conditioner is SO important. Soap residue will dry the skin and can lead to excessive scratching that can cause problems. You mentioned you clean the ears weekly. If they are dirty every week, then you may have a problem. Healthy ears, IMO, should not be dirty every week. Even though u are feeding name brand foods, it doesn't mean they can't cause a problem for your dog. Although, if she is just scratching in that area, it's probably not the food. I like a quality oatmeal shampoo n conditioner. I currently use Pure Paws.
Being a good detective will help you zero in on the problem. Once you know what you're dealing with, you can go from there. Good Luck.