Hi
It sounds like it might be a fungal infection, this often causes itching in dogs, they call it Queensland itch or summer itch down south. I've just tried a new treatment that worked wonders.
Its an anti itch treatment called Thoroughbred Fix. Its a treatment for fungal infections like Queensland itch, dermatitis, eczema, mange and the like. I used it on my terrier who had Qld itch all over his back, nothing else I tried worked but this stuff cleared it in about 5 days, amazing! It even helped his hair grow back a lot quicker and after about 12 days it was like he had never had a thing wrong with him. You can only get it online at www.thoroughbredfix.com.au and they have some really heavyweight industry endorsements too, so it must be good. Give it a try! I found it incredible.
Steve