This is what has worked perfectly for me:
Ordered birds medicated for Marek,
Birds arrived - dip each beak in sugar water.
FIrst day - feed available all the time Medicated crumble.
Brooder 4' x 3' 30" oval (no corners) up off the floor covered with cloth to keep heat in and draft out. Removed after 2nd week.
Once birds eating feed raise it up into chick feeder above their backs which is available all the time; then added aspen bedding (changed weekly).
Fresh water quart size also raised to the chick back level and changed several times per day... Water untreated well that we drink from. Have water tested! Might solve problem at least eliminate one variable.
Covered at night and maintained temperature decreases at 5 degrees per week. 100% survive.
Not to be smart, but I listened to people that have successfully raised chicks and followed their advice. My goal was to raise healthy chicks. You have far too many negative possibilities. And it seems as if you are assuming your process is not the problem.
If you continue to do what you have done in the past, you will continue to get the same result.