I believe that sometimes, summer raised pullets will not come into lay until later than those hatched in the early spring. They might start laying in December. Right now, the days are getting shorter, which is not biologically stimulating to them. They might wait til early spring before they commence. That is the reason some people ( I do not myself) will put lights on in their coop to get the birds thinking the days are getting longer or long enough. There is probably nothing wrong with your chickens. I would not give them the calcium carbonate containing layer feed until you start seeing some eggs. But otherwise, just keep them healthy and they will commence laying when nature tells them.
Layer pellets will not promote laying. It is just extra calcium to protect their bones during egg production, I am told it can be hard on the birds kidneys in general. I don't know if that is true. I would give them what you feed meat birds til they start laying. "Flock raiser" by Purina is good stuff. And mites, while they should be prevented, will not cause chickens to delay laying unless the infestation were such that the birds were anemic. As for worming. Every six months is more than enough. Worming solutions are toxins and you are essentially giving your birds a low dose of poison every time so you have to be careful. The long and short of it is if your birds look good, and behave healthily, they are likely healthy.
I also have 3 pullets, leghorns that are 8 months old. February chicks, And I started getting eggs from 2 of the 3 at just about 5 1/2 months. I got 2 eggs a day for 3 weeks then nothing for about 40 days then 1 egg for 3 weeks. And nothing for the last 40 days. I have. I bumped up to a 21 % feed about 20 days ago. I have 1 ISA brown hen that lays all the time even when we hit 121° here in Phx she kept laying. she stopped for 2 weeks in august after I had to rehome the rooster ( she loved him) but is doing great on the 21%. Nice big eggs with great shell. I did notice new pin feathers on one pullet but the other two are not in a hard molt however on all three of them the combs have shrunk and are no longer red they are pink now. They eat great and always have full crops and feathers are shiny. I also have some April pullets, so 6 months but they are heritage and I'm not really expecting eggs from them any time soon. Sooo I'm kinda at a loss to
