Check pH run off, and try giving a flush anyways. Coco holds salt real well and can get lock out from out of nowhere. Definitely does not look like it is brought by s, ca, mg, or fe. I also assume it is lock out because the ones up front look like they are borderline over feed with N, while the ones in back look borderline underfeed.
Are these growing outside, or were the brought outside just for the picture? It could be light. If they are shaded most of the day, it could be because the ones in back aren't getting enough light, all shade may look the same, but the closer you get to the unshaded part the more lux you could pick up with a light meter, even if there is no notable difference with the naked eye.
EDIT:
I notice that the ones in back are all in smart pots, the ones up front are mainly in hard cases, not only that, but the ones in back are taller and bushier, or at least looks like it. So what kind of def could cause node spacing to stretch?