Zande’s recommendation is spot on for a healthy dog who is simply picky. I cried myself to sleep many times over one of my dogs. I was convinced he was going to die! He’d like a food a few days and then stop. I mixed everything into it, tried raw (he hates it), tried every protein, every type of food, etc etc. Same results!!
The only thing that worked was finding a food he liked and I felt comfortable feeding and stuck with it. Yes he’d skip some meals but at 5 years old he now eats his food with very little issue. You REALLY have to practice tough love and sick to it. If your dog is healthy it WILL work!
For the record, I feed Fromm and rotate flavors. He does not do grain free as the good carbs help keep weight on him.