I had the same thing happen with my first bunch. (lost 9/25) I think they were heavily stressed and cold from shipping. It was 38 and raining here and below freezing in IA when they were shipped/arrived.
No expert by any means but here is what I suggest.
Get it a little extra warm in the brooder, say +5 degrees. Put a divider in the brooder and put the small/weak chicks in one side with plenty of food and water and the rowdy big chicks in the other side. They cant handle a lot of roughness the first couple days if they were left in the cold during shipping or shaken up much.
Try some plain yogurt mixed with their chick starter, say a tablespoon to start.
Check them all for pasty butt and wipe as necessary.
I think that by getting them a little extra warm it takes some of the stress off their bodies trying to stay warm. It also makes them sleepy so they can recharge their batteries instead of running around freaked out by the new environment.
Was it cold during shipping or when you got them home?
Good luck, hope you get them under control.