I looked more closely at the picture - can't see their feeder or waterer - is one of them that grey thing at the cooler end?Thanks for clarifying, it's a bit hard to see. I would still take the light to one end and point it straight down to just create a warm spot, not a whole warm brooder half.
Other areas to make mistakes are type of feed and age of feed (spoils within weeks), not feeding grit, dirty feeders and waterers, wrong bedding material. If all those are good, really that's all you can do. If you spot a chick that seems droopy you can give nutri drench (best) or Save-a-chick, or at least tiny pieces of raisins and make sure it has easy access to water and feed and isolate if need be - and then just hope for the best. I've had it work and I've had it not work, at different times.
And just a few weeks ago I had a healthy looking hen just fall over dead out of the blue. She was just under a year. Most likely she had egg yolk peritonitis - which has no cure - but I didn't do a necropsy, I just wanted to bury her since she was one of my two favorites out of all 15. These things can happen and it doesn't mean you have necessarily done anything wrong. Someone here at BYC said "where there's livestock there is deadstock." Caring for animals is tough!