-How old is your duckling? For the first 2-3 days they are still stretching out from being curled up in the egg and tend to walk a little wonky for those first couple of days.
-What are you feeding them? If you can make sure they are on a duckling specific feed so they are receiving the correct levels of vitamins particularly niacin.
-I would switch off of the brewers yeast and if your feed does not contain enough niacin for ducklings use a B-complex that can be either given in their water or via syringe.
I always start my ducklings on a duckling specific starter feed (I've been liking Mazuri for starter feed lately) and immediately on Durvet's Vitamin and Electrolyte mix for the first few days (it contains niacin which not all electrolyte and vitamin mixes do since they are often mainly geared towards chickens). After that if I have a duckling that seem to need a little extra help I give Poultry cell in a syringe or I will add it to the water if there is more than one that needs a boost.