I think I see purpling on those leaves. That's classic phosphorus deficiency. Read the below and look at your plants to confirm it. If they're low in P, they are probably low in everything else, too. What analysis fertilizer (the three numbers on the package, like 10-10-10) are you using?

Deficiency and Toxicity Symptoms


Phosphorus - P
Deficiency
Lower leaves and stems

First sign of deficiency is stunted plant growth, and leaves become darker green. Then, lower leaves and stems may develop red or purple coloration. In worst cases, lower leaf chlorosis and necrosis occurs.