"Evening" is a vague time reference, so not sure if you mean dusk/dark or just sunset. Mine don't go into the coop until almost full dark, some going in a little earlier but most will stay out until it's hard to see clearly. If yours aren't used to coming and going in that outside gate on a regular basis, they can get confused about how to get back into the coop and walk around and around that fencing until it's too dark to see and then they'll just roost anywhere they can before it's too dark to see a landing point.
This is the time of year when egg counts can drop due to readying for molt, so the egg count decrease could be due to the heat and molt or they could have established an outside nest in which many are contributing towards. That's one point of free range that doesn't go away....at that point, one can just confine them to the coop for a week to retrain to the nests and also look for their outside nest~most likely inside your barn...could be in an old tub, on hay bales, behind equipment, anywhere...and either let them keep it and collect eggs from that, or destroy it, at which point they will just establish another....BRs are pretty bad for developing outside nests. Of all the breeds I've had, they were the worst for doing that.
It would be harder for you since you have a barn structure, which is pretty much like a huge coop in their eyes, especially since their coop is located in the barn...they just see the whole building as a coop and nesting site. That will be tough but once you learn their habits, it could still work.
