Seems to me like you are doing a pretty good job of taking care of him...... Ducks need Niacin (a vitamin) in their diet, and are very sensitive to how much they have. I would recommend going to a feed store and getting a Duck/waterfowl starter/grower( if you haven't already) . Also, Ducklings...
Step 1: get some millet.(or other treat) and slowly put your hand in the cage by her until she eats it. do this for a few days.
Step 2: get some of her regular food in your hand and do the same as with the treats. (you may find she is more reluctant to eat this.) do this for a day or two...