It could be a lighting issue. It's winter, and they don't get enough hours of daylight to produce many eggs. I'm not familiar with the breed, so I don't know how good of layers they are during summer hours to compare, sorry. Some breeds continue through winter where others almost completely shut down.
It could also be that someone is eating the egg.

What do they get fed? If they're deficient of nutrients, they may stop laying, or they may start eating their own eggs to fill protein requirements.
Last but not least, but most unlikely, is brooding. It doesn't sound like the case, though. They don't brood unless they have eggs to sit on, so I would think that one is highly unlikely.
I would assume its just because it's winter. If that's the case, she should start back up in early spring, or as soon as it hits 14 hours of light per day. You could try supplementing with artificial lights in the meantime if you want, but IMO, they like and need the little rest from laying for a bit.
