Water them with a high P/K nutrient solution with a low PH of like 5.5 - 6 and you should see some improvement..your PH is the answer, obviously.

If you have access to General Hydroponics 3 part Flora series use the Lucas Formula of 8 ml micro and 16 ml bloom per gallon of water, maybe start at 50% strength in thats same 1 - 2 ratio and see how they respond. The PH should be just about right for your situation without any adjustment if you start with clean RO water. I would water them until it runs out of the bottom of the pots and catch some of that runoff and test it if possible..ideally you want it coming out lower than 7 or yellowish green color using the test vial with indicator drops.