Questions are very welcome. And thank you for the thorough answers.
It appears your setup and feeding is just fine. Heat should be adequate for all chicks. So it boils down to these little laggards. Yes, finely minced hard boiled egg is easier for tiny chicks to handle than larger morsels. And...
First, your location would be most helpful.
Have these chicks been on local soil?
What are you feeding them?
How large is your brooder (dimensions)?
What is your heat source?
How warm is it in the brooder on the floor under the heat?
How warm is the room you have the brooder in?
Get some...