I would say; let the chickens have their well-deserved break/rest. They aren't machines and if you were wanting eggs. Get Leghorns! They lay year round without light although they can be unreliable in the winter. I understand that you need to sell the eggs. There isn't really other explanations besides that it is winter and chickens stop laying then. They need about 1 month-2 months rest before they lay again. If I were you and I wanted efficiency, I would let them rest in the winter so that way they lay for more years. Right now, you are making them force lay in the winter which is not good as they need a break once a year. In the end, chickens are not machines and they need their yearly break.