Just a thought, have you raise the feeder a little higher? To the height that only their beak/head can reach it to peck and eat? My chickens used to do the same thing until one day I was @hardware store and saw some wire cable with adjustable fastener/clip for less than few bucks and used them. All my feeders are now hanging with these adjustable cable and rarely see scattered feeds.
You can also use rope since you probably have them hanging your feeder already just make it higher than usual.
You can also use rope since you probably have them hanging your feeder already just make it higher than usual.