Usually when they are younger, in the absence of older birds, they tend to stick close to the coop if not in it. Mine were hatched in late February and they are just now venturing out of the coop, and it's only because they are following the adult trio out and about.
Maybe you just need to give them some more time to grow up and become more courageous. Enjoy their little timid selves while you can, a few more weeks and for the rest of their lives you'll be running around trying to catch them again.
They'll come out on their own time. You can encourage it with treats, but for the most part they'll be timid and shy for now.
-Kim