Your ducks may be burying their eggs or rolling them in the mud so they're camouflaged. I've been known to stand right by an egg for 15 minutes while I'm out playing with the chickens and ducks before I finally notice it's there. If there are any piles of leaves, grass, straw, etc; check in the piles. I also had a duck that found a spot under a small bush pile where she dug a hole and buried her eggs for several days before finally I saw her going in there and figured out what she was doing. There were several eggs buried under some dead leaves, and the ground looked completely level and undisturbed because she covered the hole so well. My ducks have all been generally good about laying purposefully in a nest, but the downside of that is they do hide their nests well.
Has it been particularly hot for the past week or two? They might have stopped laying due to the heat. My two girls have stopped laying for the past few weeks as well, but they're molting. Yours may be a little young for a molt, but if you notice they've lost a lot of feathers recently, that could be the cause.
