Could be alot of things. Like you say it might be the cold weather, hens, it also could be alot of pressure or they're call shy. The season there has been in for a month now. What I would do is go there in the morning and set up and just cluck softly and sparingly as in every 15 to 20 minutes. Sit tight for a couple of hours. You never know, something might come sneaking in to you. Try it, what do you have to lose.