ok take a look at the plants! number one has curle...

Hi friend! Curling up can happen due to heat stress, high humidity levels or root problems, like over watering them. Nitrogen deficiency makes the plant look pale, lime green. Try to find some lower leaves that look too yellow and try to pull them off, if it's hard to pull off, no nitrogen deficiency, because leaves use to fall for their own when it happens. They should not look pale or lime green, when it happens plants use to have some problem and they grow slow. Like you have said, numer 1 is growing slow than others, so yes, maybe you got some nitrogen deficiency bro. It maybe could be some iron problems, but I dont think so.. but check for yellow new leaves too. Iron just affect the new growth. Like you said you have raised the light, try to raise the nutrients too, maybe they just have used up all the soil nutrition and need some more now. Happy grow!