Gosh, mine tend to go for the corn most of all in scratch, but I RARELY feed it any more. It's really not that healthful of a treat, so I've pretty much "weaned" them off of scratch alltogether. Maybe their tastes are changing with the seasons?
I quit feeding three-grain scratch precisely because my birds would turn their noses up at the cracked corn. Eat all of the wheat and milo, but not the corn. I suspect it goes off much faster than undamaged grains. I feed whole corn and wheat now.