Cat Food Quandary

[Kookaburra]

Please help. My kitty is losing her hair -- mostly from her butt and belly, but a little on her face too. The vet said it's an allergy (to what? don't know), and suggested giving her cortisone shots. They worked at first but not so much anymore. I feed her Eukanuba Lite chicken flavor. Is there a cat food out there that is helpful in clearing up allergic reactions/skin problems? My vet hasn't been very helpful in that arena, and the poor little sweetie looks awful.

Thanks in advance.