Beautiful adorable ducklings
@Cayuga momma
No, no, please do not float them in water. Too many false negatives and if they are alive, you should be able to candle and know if they are dead or alive.
I'm not sure what breed your eggs are, but usually duck eggs are even easier than most chicken eggs, so maybe you are using too weak of a light for candling? 200+ lumens do the best, the higher the lumens, the better. 400...500 is great.
I assume you can at least see the air cell. Look directly next to the air cell for any "clear" gaps between the body and the membrane. Also look for the air cell to be out of shape, from what it was the first 25 days. When they get into hatching position, the air cell will "draw down" and extend into the fat end of the egg.
Try taking some pics to post, and maybe we can see what's going on.