For posterity, I wanted to post an update to my reply/advice here-
Our older lady ducks are 22 weeks old and up to 3 days ago they weren't laying. We started giving them a little dry cat food over the weekend with the regular feed and we got our first duck egg on tuesday and another yesterday. They are half mallard/half rouens. I'm not one to give advice I wouldn't also try. I saw the tip on Goldshaw Farms, also read it a few places.
Also worth nothing, 2 of our easter eggers also started laying little blue eggs the same day. Maybe it helped them, too?