hmmm, I could be wrong, but you said you had them in cups only? Root bounding is easier to spot because it starts at the tip/top of the plant and works its way down...the leaves don't get brittle or anything, just yellow...if you have them in cups and there on the 3rd set of leave I could see this possibly being the problem, transplant them to a gallon container, then again before final flowering.

You can check for root bound by letting the soil dry out a little(not completely)squeeze the sides to loosen the dirt, turn it upside down and slide it out(I use two fingers to brace the stem as well)...if the soil is compacted together its root bound, if there's still loose soil in the cup and it starts to fall apart when you try to slide it out, its not root bound...