If your birds are 18 months old they may well have stopped laying to molt,'
if you live in the northern hemisphere.....
..or are laying out in their range area due to the influx of 'strangers'(the newest chicks).
Free range birds sometimes need to be 'trained'(or re-trained) to lay in the coop nests, especially new layers. Leaving them locked in the coop for a week or so can help 'home' them to lay in the coop nests. Fake eggs/golf balls in the nests can help 'show' them were to lay. They can be confined to coop and maybe run 24/7 for a few days to a week, provided you have adequate space and ventilation, or confine them at least until mid to late afternoon. You help them create a new habit and they will usually stick with it. ..at least for a good while, then repeat as necessary
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