several of my hens stopped laying

So, I've got 7 newer hens that I've added to my existing flock of six back in the spring. They're approaching nine months now. They all started laying sporadically between 5 1/2 and 6 months and quickly hit their stride, each laying daily. Suddenly, at 7 months, two of them stopped. completely. Not a single egg for two months now. A third bird gave me a few more and then followed suit. The other new ones continued to lay away daily. With the shorter days, production is waning in general, but all the others are still laying regularly. I've caught the non-layers in the boxes on occasion, but never an egg.

Any thoughts as to why this might happen? They're too young to molt, correct?

My run is pretty secure, i don't think it's a pest issue. I've spent entire days in the vicinity (my garden is right there) keeping a watchful eye and have never seen anything. Likewise, i don't think hens are eating them or I'd have noticed. I've come to the conclusion that they've just stopped laying.

They don't appear stressed, They're midway in the pecking order. I did end up with two roosters instead of the one i was hoping for, so I've got a little more rooster than i need for 13 hens, but again, no one seems stressed.

Is this uncommon? Any ideas to get them going again?

What breeds are the non laying birds, and are they the same breed as the birds that are still laying?

All three araucana. they were giving me beautiful green/blue eggs, the only reason i know it's them. the other new ones are barred rock (2) and buff orpington (2), and they are laying away.

Araucanas, Ameraucanas, Easter Eggers have all been selected more for egg color instead of maximum production. Generally they do not lay as many eggs a year as the high production birds, but may continue to lay for a greater number of years. I think what you are seeing is nothing more than a breed specific trait.

Ok, thanks. I guess i had read that they wouldn't be quite the producers that the other varieties i got were, but between the three of them, not one egg in two months? I thought maybe something else was going on...