That's why I always move my chicks to the floor the first day because I have lost them like that.I take some wire fencing and form a circle with it. I put my momma hen and chicks in it on the floor of the coop. I put food and water within easy reach.
I have had chick jump out of nestboxes. I have had chicks go to far away from mom and can't return, and I've had other hens attack chicks and mom. It's best to let everyone become familiar with each other through a fence.
I keep mine separated in the wire circle for 1-2 weeks until all chick can keep up with mom, understand her calls and can get away from other hens. Usually after 2 weeks everything works out fine.