Has anyone seen this? No other birds seem to be affected. I have removed him
/her to avoid being picked on.
i would go to or consult over the phone with an avian vet or exotic vet immediately, that is if u care about the duckling ( Not saying that as offensive, its because some people just cull them :( its really sad ) But if he/she is a pet definitely go to the vet its very important that you discuss his/her diet as well as have some blood taken, he/ she could have a defiency that they can fix by just a shot or a neurological problem that they can discuss with you about.
Could be a niacin deficiency also Do you give any Brewers yeast along with the vitamins. Pekins grow supper fast and it maybe your lil one just needs so extra niacin to help it through this rough patch. I'd get either nutritional yeast or Brewers yeast most health food stores or Whole foods probably carry it 1 tab to 1 cup of feed. If it is a niacin def you should see improvement in as little as 3 days but may take up to a week. Worth a try.
If a vet is affordable to you by all means go that route.