Bought 4 more hens 6 days ago from another seller and only one hen is laying. (3 leghorns and 1 RIR. They claimed to be 8 mths old.
Hi, hope you are enjoying BYC!
Lets see pics. full body including legs and head/comb.
What breeds and age are the first flock? Not mingling is normal for new introductions on that scale. Eventually they may.. but first there's some dispute to settle with the current bonus being that both groups are new to the space and may not have any established territories yet, softening the integration process perhaps.
Possible hidden nest somewhere in the new free range space/time?
Ah man.. if I could tell you how many times I had an 8 month old bird "mini" molt..

due to genetics or some other reason.. My feed routine is dialed in, so it wasn't nutrition. Parasites are always monitored and treated if needed so it wasn't that either, nor weather as my climate is mild. nor daylight due to it being mid laying season.
As noted by another poster.. it can be more than a month before returning to lay in these instances. Feather and shank condition as well as toe nail length and a few other things may be able to point to whether or not the stated age seems to be accurate.. the reason I asked for pics.
I have also had some fresh pullets lay gang busters every single day their first lay season for 21 days in a row with only one day off for about 2-3 cycles and then taking a sudden 3 week break. Eventually their system quit hiccuping and the regulated laying ability was on par with their genetic (breed) predisposition averages.
I'd probably give it a month before making any drastic decisions like dinner.. which is also okay.. but pullet and hens usually go for way more than a bird is worth on the table here.