Laying issues come up sometimes, and are not always easy to diagnose. Try to be patient with yourself and the ducks.
It seems to me that some ducks are more prone to nutritional deficiencies. A diet that works for one may not be optimal for another. Also, sometimes I think that ducks that appear fine could actually be doing better given a more appropriate level of nutrients. But few of us can afford to do expensive and extensive blood and fecal tests!
As long as they seem active and in good feather, not too skittery for their breed, I wouldn't lose sleep.
Try some poultry vitamins-electrolytes-probiotics. You can get them at a feed store, often. Online you can find them, too. Calcium and vitamin D as well as protein are essential, so read labels and try to figure out if they could use more of any of those.
My runners are taking a vacation from laying right now. That's not all bad - they are good layers and after almost two years need a rest. I have introduced two new ducks to the flock and this is the first time we've added members. I also slightly reduced their protein intake by cutting back on the amount of cat kibble added to their diet as a supplement. Those two things, as well as it being winter, are reasons for a laying hiatus.
Sometimes ducks can get stressed by things we don't notice. Could be a dog or raccoon started hanging around too close for comfort last August. A low-grade infection could be the problem, too.
What breed do you have? Some breeds are more seasonal layers than others.
