They are not going in the coop. They r all clustered together in the corner of the run. The un is fully enclosed but i am wondering if they're cold? They're fully feathered and it is 78 degrees out right now.
I would make them go in their coop and set them on their perch. Safest place for them to be at night. Use treats if you have to, to draw them inside. We do that to this day if they are being stubborn about going inside. They're always suckers for snacks.
If they are used to sleeping with a heat lamp it might be helpful to hang one in a corner of the coop for a few days. If it’s too warm just use a low wattage bulb. Might help to train them as to where they should sleep.
When the sun goes down they get chilled and cuddle together, and like a previous poster said they can kill who's on the bottom. Chicks do not automatically know how to go in the coop at dark, they need to be taught. So until they do it on their own you will need to put them there.