An older layer is often a "trainer" for the younger pullets. In the absence of an older hen, the pullets have absolutely no idea where you want them to lay. None. Thus, you basically have to work with their instincts. Birds are ground layers. Chickens like to sleep in the boughs of trees to roost, but they never, ever, go broody and build a nest up there. I've never seen that.
I have seen them lay down in basement window wells, down under decks, etc. Their instinct tends to make them think of the ground. Thus, we go with that. We put our nests very low, just a box set on a half block, 6" off the ground. They take to this very, very well.
They like dark, private laying too. Face the boxes away from light sources. Some folks even use strips of cloth to make a kind of curtain for privacy or darkening. These things do work. Are your present nests up high?