A few other duck owners are having the same problem. Nobody is sure what is going on. Have you sprinkled brewer's yeast on her food? Sometimes the extra niacin helps.
Waterfowl need more niacin than what is in regular starter. If you use medicated feed, thiamine will be blocked (amprolium is a thiamine blocker because cocci feed on it). You might make sure you are using non medicated feed for this one and add the brewers yeast and continue the 2 drops of Poly Vi Sol (without iron) in the mouth twice a day for a week and see how he gets along.
Where can I get the niacin at and how much do I give of each? The other duckling with her is maybe 2 days older and three times her size. It's like she's barely growing as well.
Crushed Brewer's Yeast tablets (in the vitamin section of Wal-Mart) over the food can give her the niacin she needs - as far as Poly-vi-sol, I gave my abnormally small, not growing mallard duckling 1 drop in about 1 tsp of plain yogurt with a little honey three times a day, as well as adding a few drops to the brooder waterer. She didn't start growing until I also added live worms to her diet.