I have some questions to make things clear.
How are you mixing those nutrients? Are you controlling EC?
Are you also giving 1,5 liter of water for feeding too?

My thoughts,
1,5 liter water looked not enough for 3 gallon pots to me. Probably water isn't going equal in the pot. Only top half of the soil is getting wet enough I guess. And if you consider most of the roots are in deep, here we can find something connected maybe.
I'm in 5th week too and giving 5 liters of water/food to 3 gallons. And waiting until it gets dry (4-6 days. Usually they drink out everything in 3-4 days.) Yours are getting only half of it.

I don't know much about PH things but I don't agree with Weezard here. 6.2-6.8 is ideal I thought. 6.5-7 looks OK by that scale. 7 is a bit high but not "too". Also consider, PH solution you used for monitoring so it can be little less or more and half point can change everything.

All I said but I'm nothing more than Weezard here, so wait for more responds.
I'll hang and learn too