Can they actually get to the food? I've seen weird issues where it looks like they ought to be able to eat but for some reason they can't path properly. Try picking up their food and placing it down again.

The limit is, I believe, 10 chickens within 10 metres Bear in mind they do move around, so I guess it might possible a few eggs got laid while all the other chickens were at one end of the pen? You may have to kill a couple more to make sure the average population density remains low enough to reliably lay eggs.