I would place my money on either iron or manganese. I would foliar feed with a kelp product to cover all your micro nutrient bases. You may want to give them a watering or two at a slightly higher PH as well to get a little more mag uptake. (higher than 6.1).
Also, if your doing the 24 on light thing, switch to 18/6...or at least I would.
usually Phos, or over-watered/cold......or genetics. if the leaves start to bronze, its Pi was thinking (P) or (MG) but i cant tell for sure