HELP Diucklings dying

[wheezie]

3 week old ducklings. Have lost 2 in the last week. They were find in the morning then when checked midday they were dead. It has been very hot so thought they might have gotten overheated, but this morning one was not walking. After I held it it seemed ok. This also happened with the 2 that died several days before they died. They sat and then struggled to walk, after I held them to look at them they stood up and walked and seemed fine until they died.
They have plenty of shade, water, clean food. Mom is with them and is very protective. The ducklings are Khaki Campbells, Buff and Cayugas. Mom hatched them all.
Any suggestions?