Animallover, you've received lots of good advice so far, but I must say everything you've described is very normal for a developing duck. The vitamins will give your bird and extra boost, but if you are providing a well balanced diet for this bird, you'll be fine. You've not mentioned anything that seems out of the ordinary.
Please give it some time and I'm sure you'll see improvement shortly.
Please give it some time and I'm sure you'll see improvement shortly.