The only thing I can think of is that the sire, being an Ameraucana, may have passed on possible later-laying genetics. Some Ameraucanas lay a little later than that some other breeds. I've heard "8 months" bandied about a lot when it comes to Ameraucanas and their "cousins." That's the only...
Well, there is that "sticky" article at the top of the Chicken Behaviors and Egglaying, but you've probably already read that as many times as I have.
When I have the problem you do, which I happen to have right now ... well a few more should be laying than are ... I pull out the big guns...