This is a tough call. However, since the affected plants are clones taken from the same mother plant, and I assume that you are using the same watering, light and fertilizer regimen on all of the clones, then some of the obvious causes can be ruled out. In order of first guesses, I would rule out; fert burn, MG toxicity, over-watering.
So, as I look closely at the pics, my best guess is some sort of bacterial leaf spot infection. That makes sense since it is isolated to some of the plants, may have been introduce by handling or animals. The bad news is that if it is Pseudomonas spp. then there is not a lot you can do. The plant may recover, and you can try daily misting of Neem oil. But by all means, try to handle the infected plants AFTER you touch the healthy plants, not the other way around, to avoid spreading the infection.
Sorry I can't offer a more optimistic prognosis.
VM