I'm thinking it's the Ph as well.
But since you said your water Ph is 6.4, I would question the soil Ph.
Is there sufficient runoff water when you water?
If you don't have a soil meter, you could just flush your soil (Ph'ed water)and that should bring your soil Ph in range.

__________________
L.O.W. T.H.C. starts with an imbalance in one of these;
Light Oxygen Water Temperature Humidity Co2 My Youtube grows