I have found that stress reduces egg laying. Stress can be caused by moving, change in the flock makeup, new animals, heat, cold, diet, injury, and more.
My runners are good layers. They took about five months off after I introduced our first drake. Then they got back to laying.
They also are turning white. They were solid black or chocolate, and then slowly the snow started coming in. They are beautiful.
So, I would look closely at their diet, making sure it is nutrient rich with a good balance of protein and fresh vegetables and vitamins and minerals. Ducks can have greater needs for some nutrients. Each duck is different.
I would make sure their environment feels safe, and the air quality is good (ventilation, bedding management, etc.)
One forum member had a hose that leached chemicals that suppressed egg laying. That is about the most unusual situation I can recall.