They are a bit young to use a guillotine style feeder like the Rent a coop feeder but if their weight is enough to raise the lid, it is just the fear of that overhead lid that is stopping them.
And I don't blame them. Imagine your front door hung sideways and you having to step on a treadle and lean in under the door to eat. You might pause at that ....
Maybe they have experienced the lid dropping when one of them jumps off, might take two or three to raise the lid and they just don't trust the feeder.
If it were me, I'd use a different feeder or let them eat with the larger birds when they can until they are big enough to operate on their own. Blocking open one of those guillotine style feeders is always a bad idea as it teaches the flock that the lid isn't supposed to move when they step on the treadle. And three week of open feeder when most people bought the contraption trying to solve a rodent problem.
The real purpose of a three week open feeder training period is to get you past the 30 day
Amazon return period. After three weeks you remove the training bolt, find out your flock refuses to use the feeder, and you might mess with it a few more weeks before becoming frustrated and trying to return it. Most people are so nice, they blame themselves, and the 30 day return window is past.
Don't buy these old design feeders. Twice the cost, half the feeder, made in China, good luck ever finding parts once they need them.