So, you are mixing corn with your feed? Corn is tasty, but lacking in nutrition.....like, say, potato chips are to most people. The chickens are doing what would be expected. A simple solution is to offer feed, and only feed, in feeders and save corn for an occasional treat tossed to them outside if the feeder. Withhold corn, and other treats, altogether for a week or more to get them in the habit of readily consuming their feed ration.
Also, check the feed...smell it and, if you have the bag still, the mill date....to assure freshness and that it is not spoiled