Have you tried any other meds? I've given my Corgi Clairiten(sp?) before, vet said at his weight (36lbs approx) half of a tablet. They also said if that doesn't work I could try half a tab of Zyrtec. Right now he's back on Benedryl, it's the only thing that is keeping him from licking his feet and gagging after, but it makes his terribly grumpy, which is hard to believe with his regular attitude anyway.
Are you still giving the fish oil, even though you changed foods? You could also try (probably going to be a very unpopular opinion about to be written) putting one raw egg over her food a day. We usually end up with someone pecking or sticking a toe nail through one ever day here, so it just goes to the dogs.