This is not a duck story but might be helpful. I just hatched out a little black Japanese chick. It was pipped for 3 days before I finally helped it out and then it was real weak and just laid there. I left it in the incubator over night and the next morning, unfortunately, it was still just laying there. I dipped its beak in water and left it in there until I had to go to work that afternoon, then I felt I had to put it in the brooder with the other chicks. He couldn't stand and kept falling over and I didn't have much hope for him. By the time I got home at 7:00 he was up on his feet and walking around good with the rest.
Just have to give them a chance to get going. If your duck looks like he is getting worse, I would get his beak dipped in water.