khaki campbells egg question

[WendyK]

I have 7 khaki campbell females that are 28 and 26 weeks old. However, I am only getting 4 eggs a day on average. I'm pretty sure that my two oldest are laying, with one sometimes laying 2. I'm not sure which of the younger ones is laying, but I thought by now all of them would be laying. Is it common for them not to all lay? How do I encourage them all to lay? I keep them in the co-op until after 8am just to make sure they lay in their house, but could that be the issue?

 

[chickens really]

Like everything....They all mature a different times...