You need a feeder lip extender, I mentioned one in the post above. They cost $1.50, not exactly a profit center, plus the very expensive U.S. mail shipping these days. If you buy one with a feeder they ship with the feeder at no additional cost.
Not sure what caused your hens to start digging through the feed. Maybe a stray corn kernel from the feed sack or they found a bug and thought there were more.
Until the feeder lip extender arrives, some cardboard and duct tape will work. Just make the front lip at least 1.5" taller than it is now, that ought to stop them. This is a chicken problem, not a feeder problem, but we have what is needed.