Have your males begun crowing/mounting/foaming yet?
Mine are about 9 or 10 weeks and laid their first eggs a couple days ago. They get a couple of dim lamps in their coop until 10 pm. I live in a temperate region.
So, could be environmental stress factors (maybe the cold snaps, maybe they're getting molested by critters). It could be genetics; your stock might be late layers. Either way, you've already invested in the flock up till this point, and sounds like you're doing everything right, so I don't think it's worth it to cull the entire herd. I think you'll see eggs in the next week or two.