Persistence, mostly!
Eat with and share food is a big one. Birds learn from the flock what is actually food, so they know what is safe.
Also try different prep methods. For instance, carrots can be offered in sticks, coins, diced up fine. They can also be offered raw or cooked. Warm vs cold. Try on a flat plate vs a bowl. Offer them separately (some don't like their food to touch!) or all mixed up as a salad to forage through.
Try to puree it into almost baby food!
Try mixing with foods they are eating. You can also try sprouted seeds.
If you are finding it flung about, chances are they are eating it; they are just messy creatures!