When mine were young I made the mistake of changing their food when they would get bored of it, then we got down to them not wanting the same food more than twice. I created picky dogs and it was very frustrating.
So I found a food that was healthy for them that I wanted them to eat and that's what they get everyday. there was a bit of a stand off about it for at first and then they got hungry and realized this was all their was to offer and they started eating again. I haven't had a problem since and I won't be changing their food again.