Generally, a sprinkling of brewer's yeast on top of food is what people say they use.
Niacin is also found in animal products, so if you have any canned sardines without salt and without oil, just fish in water, a teaspoon of that a few times a day may help.
Raising ducklings involves learning so much. That is what this forum is for. subhanalah has shared some good information.
We cannot rule out that she may have an infection. Are her feet or bill very warm - warmer than the others? Does her poop smell off? You may want to confine her so she doesn't wear herself out, but at the same time, isolation is very stressful for ducks of any age.
If you can dig up some worms, that may help the niacin status, also.
The recommendation for niacin capsules - not no-flush or timed release, just plain niacin - is 100 to 150 mg per gallon of water.
Do you have any B complex vitamins? I am not a vet, but if I had some of those I would use that in the water, using the B3 (niacin) level as a guide for how much to put in the water.
I cannot be at the computer all day, but will check in when I can.