Spots in leaves. Not sure

Hi.
I am a first time grower and currently in lockdown. So I am in week 5 of flowering and am noticing some spots on my leaves. The spots are only on 1 out of my 4 plants and the other three don’t seems to have any as of now. They are only on the big fan leaves as of now. I tried researching but it came down to one of the two as far as I could understand.
Its either bugs which I am not too sure about as I looked very carefully and dont see anything fishy and the spots definitely had a pattern on the leaves.
It could be ph imbalance which looks similar to this but I have been keeping my ph around 5.5-6. And I water when the top inch of soil feels dry. Usually 2 days.

Please tell me what you think it is. Hopefully its something I can manage as in lockdown I have no access to the outside world and have to use only things available in my house.

About my setup, I have 4 plants growing in coco. The strain is Gorilla Blue photoperiod. I started off with AN ph balanced line. But with the lockdown and having run out of nutrients( didn’t realise how much I would need) I am using water soluble NPK with CalMag since the last month and havent seen any issue. Also added couple of photos of current state

Have you checked the PH and PPM of your runoff to see if it's in line with your input PH and PPM?

I used to check it initially for 3-4 weeks and it used to be fine. PPM would be a little high as sometimes the coco wouldn’t dry and I had to wait 3 days to water them.
Run off Ph used to be high initially so i moved from 5.9 to 5.7 for my input which bought the run off to around 6.5
I havent checked in a while but tomorrow ill water and will definitely check

Yeah check those out mate and come back, your PH might be off but you could also have a salt buildup in the coco causing a lockout, you watering to a good amount of runoff each time?

Yes. I had read 10-15% run off. I start of with 14 litres for my 4 plants and usually i collect around 3 in run-off

But thanks for your quick replies.

Turns out it was my PH. Today i calibrated my PH meter and it was off by 0.4. So i thought i was giving 5.7 but it actually was 5.3