One of the most common reasons, particularly in late fall or winter, has to do with light.
Most chickens are very sensitive to the amount of light they get and need 14 hours of light in order for the egg laying hormones to be activated. It does vary some by breed, but 14 hours is a pretty good target.
As a point of reference, on January 25th on our farm, we’re getting not quite 11 1/2 hours of light that’s bright enough to make a difference. That’ll increase a little each day up through mid-June, when we’ll be getting about 15 hours.
Recently this winter, we’ve been moving breeding flocks from the barn out the the field. In the barn, the birds get artificial light starting at 6 am, when I got out to the barn to begin feeding, and the lights go off around 8-9pm. While one of our flocks, the New Hampshires, was in the barn, they were laying about about a 70% rate, that is, I was getting about 7 eggs from 10 hens each day. But now that they’re out in the field, with no artificial light, I might be getting one egg a day. And that’ll improve as spring approaches.
If you choose to add light, first add 30 minutes of light in the morning. After a week, change to 1 hour of light, and so on, adding 30 minutes each week until you get to 14 hours of light.
Often just adding light will cause them to start laying soon, particularly with new hens that you’ve raised up to 6-7 months of age.
For some other ideas that may help, see: Why are my chickens not laying eggs?