What I would suggest is that you make a somewhat runny mix of scrambled eggs with SUGAR in it it, and some vitamin C to get their strength up.
Also have 2 water sources... 1 will be normal water (also replace and make sure its clean). And the second water source to have sugar and electrolyte solution in it.
If you are desperate you can even feed them whisked runny yolk with a bit of sugar in it, but you don't want to do that very much without cooking it.
Treating for niacin deficiency is necessary also at this point also. You could also try to see how they react to live worms, like for fishing bait that are still alive until they get better. Or ground beef, or tuna.
A lot of places are having problems with their feed, such as milled feed, etc. If they think something is wrong with it there probably is. How does the bulk of your unused feed smell? Proceed to check and act is if more than just 1 problem is in play until you have eliminated other options.
Even if the problem is niacian deficiency, their bodies are under a lot of stress and breaking down. So these other tools suggested will help to get them through the stress forces on their bodies, and deal with malnutrition in general. (Malnutrition can mean not only niacin deficiency.) Also in animals, sometimes more than 1 nutrient controls how another bonds with the body. Example; people have found people who don't eat meat, have trouble getting calcium to bond with their bodies, even if they have enough calcium. So addressing malnutrition overall will help the niacin treatments to work.
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Some ducklings will recover from niacin deficiency. Especially if you are at the 3 week old mark. Prior to the 3 weeks old mark the mortality rate is much higher.
