Do you give them time in brooder-temperature water up to their bellies - no soap - daily? They also need to be able to dip their entire heads in water to keep their eyes, ears and nares clean. Otherwise they can get infections. There are instructions for head washers in the archives.
Orca5094 just posted this in the Sick Duck? thread
Ducks must have water deep enough to dip their heads and clear out their nares at all times. Ducklings get food stuck in their nares quite frequently and if they cannot properly blow it out of their nares, it can become dried and stuck in there and cause open-mouth breathing because they can't get enough air in through their blocked nares. Do they get bath time every day? That is also a good opportunity for them to really blow out their nares.
You can also make them a deeper waterer by getting an old plastic food container with a lid. Cut a hole in the lid just big enough for them to get their heads in so they can wash their eyes and blow out their nares. You don't want the hole big enough for them to get their bodies in, though. They will just get themselves and the brooder soaking wet and will chill quickly, which is life-threatening to little ducklings.