Their bodies need to rest during the shorter days of the year (lighting beyond natural daylight hours may wear hens out sooner - I don't do it). It is natural for them to take a rest and it's good for them to rest. It is not something to be upset about. This will make them stronger and able to resume more active egg laying in the spring and summer. You will find a lot of information about this by reading on this site.
Of course, all that said, make sure they always have really good and fresh nutrition as this can affect the ability of their bodies to do the work required to make an egg.
JJ
And p.s., as noted above, during molting (and for a time before and after) they don't lay much if at all then either. Molting uses a tremendous amount of energy and there is none to spare for making eggs. They need really good nutrition during this time as well.