Why wont my hens lay eggs?

I bought 3 Rhode Island Red hens, all about a year or so old. They won't lay. I have no idea why, their shed isn't really hot, they have all fresh straw. They always have feed and water available. I also have nest boxes for them as well. I've had them for 2 weeks. When I bought them, the lady selling them told me that they are already laying.. Is there something I'm missing?