Sorry for your loss.
How big is your brooder and what wattage is your heat lamp for them? Sounds like a temp problem. If your brooder is small and you have a high wattage heat lamp the ducklings can't get far enough away from the heat. If you use too small a wattage bulb they won't get enough heat.
I keep my babies in a cardboard box (15"x24") with an open top and a 125watt bulb on one end that is flush with top of the box. They usually stay at the opposite end of the box from where the light is when they get too warm and move in closer to the light if they get cold. I keep their water and food on the opposite end of the light. The water stays cooler that way.
By the way, did you dip their little beaks in water when you moved them to the brooder? They need to learn how to drink, but they always seem to know how to eat without showing them.
I also remove my babies from the incubator to the brooder when their top layer of down is dry (they are still a little wet). I do this because I leave my eggs in the auto turner to hatch, turner is, of course, unplugged during lockdown, and I am afraid they will get their little legs hung up in the turner and also so they don't disrupt the other eggs.
I have 2 little Mallards, 1 born friday and 1 born yesterday. Another is hatching and will be born sometime today.
Again, I am sorry for your loss. Please don't give up on hatching. We all learn by our mistakes and mistakes aren't the end of the world.....keep trying.