It is normal for them to slow down in winter, and they do a molt then too. That is why they need extra protein for their feathers. Not sure what you can do about the feed where you are. Can you ask someone at the store? I think if they get more nutrition, especially in the winter they can...
In my opinion, you may not be giving them enough nutrition. Here is some information on feeding them.
Food & water: most people use a formulated chicken layer feed/pellets. In depth discussions on the pros and cons of most feed brands can be found in our Feeding & Watering Your Flock forum...