I had the same problem, it got bad too. Their hair was patchy in spots and had yellowish crusty places.
My 2 are on diamond naturals (grain free) lamb and rice and the grizzly salmon oil now. They were on totw which was fine at 1st but I think the potatoes in it made the dogs yeasty. And then we had trouble.
We have now used 4, 40lb bags of the diamond naturals I think we finally found a keeper! Good luck