They just need fertilizer.

I don't know where you are but I've already harvested and pulled out most of my corn. So you may have planted it too late.

Corn is a big nitrogen feeder, it probably needs a boost like nel said.. First time I've seen growing corn in a bucket, you may need to thin it down to just one stalk and hand pollinate to get an ear or 2, otherwise you'll end up with a bucket of crammed roots and stunted stalks.

Maybe too much watering?