Welcome!
Not really sure what you mean by the bottom of the coop.
There could be something about the coop they don't like, maybe not enough ventilation, maybe a predator -- snakes or rats, for example -- they are also quite young and may simply not have figured out they're supposed to roost or sleep in the coop. Sometimes the ramp is steeper than they like. Some people find they have to put the chickens on the roost, maybe for several nights, for them to learn. Yours are young enough that they may still just prefer the chick style of sleeping, in a huddle.
You say roost pole -- what size and shape is it? Chickens' feet are a little different from other birds and don't really wrap around a roost the same; they usually seem to prefer either a wide roost like the wide side of a 2x4, or a similarly fat branch for a roost.