Ducks rarely get parasites , they spend too much time in water for bugs to survive lol. I think you are right, molting! I once thought one of my runners had wet feather but she didnt either, she was molting . The nutridrench is a great idea, or "rooster booster poultry cell" is even better (to...
It is located on top of the base of the tail, on the lower back, just in front of the tail feathers.
You may have seen your ducks rubbing their bill in that spot to get oil on it and then they put the oil all over their feathers
You should check her oil gland, majestic waterfowl sanctuary has a great article about wetfeather and oil gland problems. Scroll down to Oil Gland
http://www.majesticwaterfowl.org/mmissue71.htm