What protein source is your current food? Lamb, chicken, beef? I'd first try to switch to a different protein source than they're now on, or have been on at any time in the past. Switch over gradually, though.
My Black Lab is sensitive to lamb, she used to have ear infections constantly! (cost an arm and a leg on antibiotics!) Then I switched her to a chicken and rice dry kibble, and poof! the ear infections disappeared! Be careful of the treats too, and what kind of protein source they have!
Good luck!