Yesterday afternoon we had an ugly thunderstorm that came up really quickly. My younger chicks (15 wks. or so) were hanging out in the big chickens' run and the big girls were out in the yard. When it started raining, the big girls went immediately to their coop, and 4 of the 5 young ones headed out of the run to their hoop/coop/tractor. My BO chick got panicked and didn't run with her sisters, and then she was so scared by the rain and the loud thunder that she just ran back and forth along the fence. I went out to the run and as I got to her it started to hail. I scooped her up and ran her to her own coop where her broodmates were already safely inside. By the time I got back to the house I was soaked, but the hail didn't get big enough to hurt me until after I got back in. No long term effects, but I noticed that she stayed really close to the others all day today.
Pretty scary for all of us. There is always one who doesn't make it with the others.
On your topic though, is there a chance that the laggard has an eyesight problem and can't see the entrance to the coop?