PH issues using jacks 321

Been having a a few plants of our Oregon Pinot and Saphire og that I think may be having a ph related problem, out of a 400 plant room it’ll be a plant here or a couple plants here, very random. (All get watered and fed the same) 6.4-6.8 ph and 1.1-1.2 ec.
Room temps and humidity and watering is all good. Running jacks 321 with weekly recharge flush. I’m wondering if they are drying out too much or if it’s a ph issue. Never had this happen before like this so it’s very weird. Specially when it’s only 15 plants or so out of a row of 100.

also yes there is a bit of a thrip problem (dealing with it) this started after last feeding

I'm new to Jack's 321 but I like it. I talked to them and they recommended to me that I use a PH of 5.8 in my hydro grow if that's a help to you. They also told me to EC higher than I was. I was running a few hundred PPM below their chart numbers and they told me to bump it up. It's been good to me this grow. If you have real pure water you'll be 200-300 PPM low of their PPM/EC chart numbers.

jacks should not need ph with ro water, it should work great between 5.8 and 6.8, these issues sound like they are in whatever medium thats being used

Why are you flushing the plants weekly?