I agree. Most of the things that I've read is that those breeds lay year round anywhere from 200-300 eggs per year. According to your location, I don't imagine they are lacking as much light as my birds are.
I would definitely look all around, maybe there is a hidden nest somewhere.
Have you checked for rats or possibly snakes? Maybe something is eating their eggs? Any issues with predators?
What feed do they get?
6 months seems like a long time for them to stop laying eggs. Both breeds mentioned are relatively good egg layers.