If your coop is too dark they will just want to stay outside as it will be lighter than inside the coop.
You could try and install a little night light using a LED cabinet light or else.
Do they have a chicken ladder to help them up to the roost?
Until they reach POL (point of lay) they will change plumage several times thus often lacking the appropriate wing feathering to reach higher roosts without a ladder and building the bad habit of piling on the ground or in nest boxes...