Yup, what katherinad said. Young ducks that reach maturity at the end of Summer/Fall sometimes won't begin laying until the days get longer in March/April. Ducks that are laying heavily need to take occasional breaks to moult, and sometimes they'll stop laying for winter.
A lot has to do with your length of days, but some has to do with the line of ducks. When I talked to Dave Holderread (author of Storey's Guide to Raising Ducks, world expert on ducks & show breeder for over 40 years) a couple months ago, he said a lot of folks think choosing the right breed of duck is the most important thing, but it's actually the *line* of duck that matters. Anyone can take a bunch of khaki campbells and make a bunch more khaki campbells, but if they're not choosing carefully for laying ability and other positive qualities, those qualities will quickly degrade in the offspring and before long you have ordinary ducks that look like khaki campbells but don't lay like khaki campbells.
Anyway--just keep feeding them a good quality diet, keep them healthy, and they'll start laying again when they're ready. If laying ability is important to you, you'll want to research the breeders carefully when you buy stock. Holderread (
www.holderreadwaterfowlfarm.com) is always a safe bet, but there are many other quality breeders in the U.S. too.
Good luck--and wait for Spring. Come April, you won't know what to do with all the eggs! Me, I started freezing them a couple weeks ago (I have a mix of mature & young ducks, and some from Holderread stock, so while mine are tapering off on production, I'm still getting a good half dozen a day), so I'll have eggs when the lean times hit mid-winter and early Spring.