Its not your PH if that is what you PH is. 6.2 - 6.8 from what I read is ideal for soil based grows.

What kind of soil are you using? and have you been feeding them this early in the game or have you been using plain water?

Because this sounds like a deficiency to me? Or your overfeeding.

I will add a chart to show what each symptom might be. (also got if from cannabis.com)

If you don't know.

Here is what each symbol is:

N-Nitrogen
P-Phosphorus
K-Potassium
Ca-Calcium
S-Sulfur
Mg-Magnesium
Fe-Iron
Mn-Manganese
B-Boron
Mb-Molybdenum
Zn-Zinc
cu- don't remember

Hope this help. Remember one important fact. Overfeeding kills your plants.

Is this your first grow?