That looks like a potassium defeciency.........because you are using good soil and good nutes I'm going to guess that either your soil ph is off or your plant needs to be flushed due to fert/salt build up or perhaps both. I seriously think you have enough k in your regimen. The curled down edges of your leaves can also indicate overnute and/or ph issues. Let me guess.......the affected plants are in the line of a fan or very close to some sort of lighting that makes them warmer. Both things seem to make a k def worse imo. (Im told low humidity contributes to a k def so that may be why.)