They love grass. They want grass, grass, and grass. Corn is a seed of grass so it's OK. But wow do they ever like .... grass. You might try doing some whet sprouts, corn sprouts, etc. I think they also depend more than many think on wild bugs, slugs, etc. I actually give mine dry cat food, a little, each day when they're not getting their "snips and snails and puppydog tails" in the wild. The only other plant they have any interest in besides grass is lettuce. There's a wild lettuce that grows here, they love it. They love domestic lettuce. It's the only broadleaf they have *any* interest in and they love it.
It sounds like you're raising young ones. From right out of the egg, they love, and eat, grass. They'll pick the little grass tips and fall on their little butts when it gives way, they are bone fide grass eaters. I'd consider if nothing else getting a kit for wheat-grass sprouting. You can find those everywhere, and wheat's a grass. Or get some lawn mix and sprout that, a few inches high when they're little and fuzzy and then let it grow bigger as they do.
Right about here I should emphasize: These birds love grass.