Ph is usually around 5.6-5.9
Next time you mix up nutes, test the PPM. If you are below 1000 PPM at 0.5 scale (EC 2.0) then I would increase the base nutrients to get there. I prefer to measure with a syringe or graduated cylinder and use mL as the math is easier.
If it's not a weak feed, this ->
thread relates to what I think might be going on with your plants. After giving that a read you could choose to either ride it out, wing it (slurry test or runoff test) and guesstimated corrective feeds on the affected plants, or if budget allows grab a good soil pH meter as detailed in the thread and see what is really going on and make a more accurate corrective feed.
Whenever I have a plant or two that gets crappy despite getting all the same treatment as others the first thing I do is test the pH of the rootzone. In DWC this is as simple as testing the nutrient pH but in mediums like yours there is a potential for the rootzone pH to be out of whack.
I used to have this stuff happen where one plant would be pissed and the others all happy and I never could figure out why until I picked up a good soil pH meter and noticed that one plant had a whack pH so tried corrective feeds and the plant responded well. It was an epiphany moment you could say.
Anyways, give that thread a read and see what you think. Maybe do a runoff or slurry test on the happy plants and compare it with that of the unhappy plant just to see if we are barking up the right tree. If you input 5.9 and it comes out 5.3 then the actual rootzone pH is likely below 5 as I have noted runoff tests can indicate the direction but not quantify the amount precisely.
Hope all that makes some sense. This is valuable advice that has helped save crops for commercial growers I talk with.