I don't know about ducks either- hopefully some of our duck people will turn up here soon. Where the ducklings like that when you bought them? Have they gotten worse since? You could try posting in our "emergencies, diseases, injuries, cures," thread. Be sure to put moscovy ducklings in the subject line.
How old are they? It is very hot now where you have them? Could be heat related. If they are old enough do you have a , swallow pan or kids pool they could swim in? Do their poos look normal? Any sign of worms or mites(on them)?
I would definitely get in touch with the seller and ask her what's going on. I know ducks can have a niacin deficiency if they are not fed properly. What are you trying to feed them?
Do you have a vet you could call for advice?