I think you're right to block off those nesting boxes at night and place them on the roosts. Soon they should get the idea....they might be low on the pecking order right now, so maybe some of the other hens are kicking them off the roost....Do you have enough space on the roost for everyone? Maybe you need to place another 2x4 somewhere and have two roosts. That's what I do....and it's funny because there are always 8 one one roost, and four on the other, and they always want to be in the exact same order......they pick their partners they like to sleep by at night.....
It'll happen soon enough...when they're young, it takes a little time before they're ready to roost sometimes. I had 4 pullets that I thought would NEVER roost.....they would pile in a corner for the longest time--even in the heat of summer.....now they are roosting just fine.