There could be many reason why you are loosing them.
What type of food are you giving them? We didn't have game bird starter in our local feed store when they hatched so they are on turkey starter.
Are there any drafts by them? No drafts, in the basement
Are they piling on top of each other and then passing? No
Is there water to close to the heat lamp? Close but not directly under
What type of heat are you using and what is your temp you have them under? They are in a low brooder box with a 65 watt bulb that gives a temp just under 90 degrees - they are 3 weeks old at this point
How many do you have together can they all get to the water and food without crawling over everybody? There were only 12 to start, I am down to 8 now
What do you do when you see them getting weak, try giving them electrolytes it's a powder that you mix with water. I can do this I have some around still from chicks