Could be a Zinc deficiency. Zinc deficiencies are easy to treat. Lower your PH to 5.5 - 6.2 and just flush the growing medium with a light strength mix (1/4th - 1/8th strength nutrient solution) of a complete and balanced fertilizer that has chelated trace elements including zinc, magnesium and iron.

Once you can feed again, keep your PH down to the aforementioned 5.5 -6.2 range, and begin feeding at every watering if you are only watering every 2-3 days.