I once worried so much about this - I would go out and place each chicken on the roost, turn around to get another one, and the first one would jump back down, and I would put another one up, and the next one would jump down. What was wrong with my chickens, my roosts, my coop???? Nothing. Eventually they all started roosting up there without any intervention/interference from me. It MIGHT be helpful to have a little ladder or something to help them get up on the roosts while they're young and haven't perfected the winged-leap, especially if the space in front of the roost is limited. (They're not helicopters). But mostly, don't worry. They'll get it.