When you say you fed it 1/3 strength, how much did you give it? There's a big difference between giving it one gallon, or 5. How long since the last feeding? It's lacking nitrogen, so if it's been more than 5 days or so since you've fed it, I'd give it some more, like 2-3 gallons of 1/2 strength,(depending on what you've been using.) then see if the foliage starts getting a darker green. Watch for the upper leaf tips to start turning brown, that'll tell ya that you're borderline overfert, and need to back off.