pH problem! Needs Ca/Mg too. Low Micro/some Macro nute problems from underfeeding and pH.
Add a Kelp extract to your feed - Age old or Maxicrop seaweed are real good ones (Micro's, B-vit's, Plant hormones and Amino acids all things that you need, that will aid in recovery)
Light leeching of the soil...Water to run through and pour another 4 cups in. When it returns to needing a watering. Water with nutrient and Ca/Mg at full strength - CALi - MAGic from GH is good stuff - You want the iron it brings to the table too. This should be pHed to 6.8 for you till harvest. (At 6.5 your right at low pH range for a cpl of nutrients to be up-taken properly in your soil/you have a soil that is running to low a pH.....DON'T bother attempting to "fix" the soil.
OR (and better)
Simply water with straight water with the Ca/Mg pHed to 6.8 for 5-7 days and restart feeding at 75-100% nutrient strength (Just what ARE you feeding with) and feed every other watering - NOT every third.....You have low micro's too.
So, feed more, add a kelp and a Ca/Mg product and pH at 6.8 to hold a better pH. (Ph to 6.5 for veg and 6.7 for bloom in the future)
Next time add around 1/2 cup of Dolomite lime to your soil (whole bag of soil and mix well) before use.