Poults need SO much protien and energy to sustain their growth, they often can't keep up with their body's demands. The dehydrate quickly, lose electrolytes, and become disoriented/dizzy. Combine this with their natural stupidity (poults often need to be shown to food and water repeatedly) and the poor poults dehydrate and starve to death.
Bigger brains later means more care earlier in life.
I had this happen with my first hatch of poults. I killed 5 of 6 before I realized they needed some "force-feeding" of sugar water. Now I keep the electrolyte water closer to the EcoGlow, and the regular water further away. I drop food directly in their nesting box, and keep a BIG trough of it at the other end of the brooder. For the first week of life, all the poults get force-fed sugarwater.
NOW-- all that being said, there could be somehting else going on with your chickies, but I do not know what. Sorry.