We free range them around dinner time for a couple hours so we can supervise.
So obviously this is not due to them hiding a nest while free ranging. The time of day is wrong.
We were getting 7-9 eggs a day by September.
That sounds like it was consistent. Some hens only lay two or three eggs a week which can contribute to getting various numbers of eggs on different days.
But we're 6 weeks out and at most we get 6 eggs a day, but often only 2 or 3 from our 8 month hens
So out of your 9 older pullets 6 are still laying in the nests. Overall production has dropped. So they have slowed down, not stopped. And many are laying in your nests, not hiding a nest.
I don't believe in absolutes for the behaviors of living animals, including chickens. And I believe that the law of averages applies to the flock as a whole. But it does not apply to an individual chicken. Just because the average age for a pullet of a certain breed to start laying is 24 weeks that does not mean that every individual pullet of that breed will start laying at 24 weeks. Some individuals will be early, some late.
I agree that most of my pullets that start laying before winter continue to lay throughout the winter and keep laying until they molt the following fall. But most does not mean all. Some don't. If the numbers add up three out of nine are not laying. That may feel a little high but with only 9 pullets just one is a pretty high percent of the total.
We were getting 7-9 eggs a day by September. We did slow introductions, supervised free range with babies and eventually got them into the coop. Egg production literally dropped overnight. Some days we do not even get a single egg.
It sounds like you did well on the integration, but something obviously happened. You can get a temporary reduction in laying when you subject them to change. They usually come back fairly quickly if it does not induce a molt. Stress can cause an early molt but you've had as many as 6 eggs in a day since. While one or two may be molting I don't think that is your main issue.
As Stormcrow points out, they can be really good at hiding a nest. That could be somewhere in your coop or run. After all this time that would be a huge number of eggs, hard to hide. I'd do a thorough search of the coop and run. You might find something.
To me, this sounds more like something is getting the eggs. Lots of critters will eat eggs. Many leave signs behind, like pieces of eggshell or wet spots. But some don't. Critters can come at different times, eating what eggs are there and not getting the ones that are not yet laid. That might explain why you get more eggs some days than others.
In Ohio (thanks for that information) it could be snakes. A snake will come at varying times of day or night and eat its fill of eggs if they are there. Then it will disappear for two or three days while it digests them. I had a 5' long black snake eat 4 eggs out from under a broody hen and come back later for more a few days later. Yours sounds too consistent for it to be one snake and unlikely to be two or three.
Canines like coyotes or foxes would more likely be interested in hens more than the eggs but does a dog have access? A dog may not bother your chickens but can eat eggs.
A human is not going to leave any signs either. A human often means someone you know, not a stranger. It does not seem very likely that a human could have been that consistent that long but like all of this that is for you to consider.
Chickens can decrease production with shorter days and severe weather. I don't think your weather has been that severe for these 6 weeks but the days have gotten shorter. I don't think that is a major part of this but it could explain one or two pullets.
I don't know what has happened to your egg production. I suspect it is a combination of things. I'd be frustrated with that amount of lost production. It seems like more than it should be.
So what can you do, other than search for hidden nests? How often do you collect eggs? Just once at the end of the day like many working people? Does it improve on weekends when you can collect more often? That might give you a clue.
I suspect you collect eggs when you let them out to free range so by letting them out you are not allowing an egg-eating predator access before you collect them. Usually I'd say leave them locked in the run for a couple of days to see if production increases. That would mean you either locked them away from hidden nests outside or locked an egg-eating predator out. But I don't think that applies to your situation. Still, something for you to consider.
I'd suggest marking a few eggs and leaving them down there. If they disappear you know something is getting your eggs.
I can't think of anything else to recommend. It is frustrating and it may be due to totally natural causes with your flock. Good luck!