I've been there. I thought I had the pickiest eater in the world. My little one is extremely picky. To make matters worse, certain foods can cause her to have colitis. So finding her food was pure hell because i also had to make sure whatever I fed her didn't cause her colitis to flare up. I finally found something we both love Merrick's limited Ingredients. Merrick's also has regular food as well so you might want to try Merrick's as well. She has been eating that for a about a year now and I prayed she doesn't get tired of it and stopped eating it.
I think the only advice I have is that you just have to keep trying different things until you find something they like. The thing that used to drive me crazy was I would find something Duchess would like and then I run out and stock up only for her to stopped eating it after about a week or she would like it and it would cause her colitis to flare up.
It's trial and error at this point but I'm sure others will probably have much better advice than I do.