I hear duck is good for dogs with allergies. And it definately will take several weeks for the new food to "kick in"
I have dealt with yeast infection after yeast infection with one of my dogs, after it got so bad that i had just accepted that is how my gurl was gonna be forever, I was suggested a food change... Its been approx 6 months now and NOTHING!

I hope the new food will help your dog out as well.! Keeping my fingers crossed for you...
Oh and another thing, be careful with treats... try not to feed the regular treats for a while, they could contain the "allergen" and you would not see any change with new food.