HELP! Upper leaves yellowing with brown spots

[Sadpygmy]

Bubblegum photoperiod in Living Soil 10 lb bags. 2x4 tent with MarsHydro TSL 2000 led lights. Water and soil both at 6.8 ph with calibrated tester. Temps and humidity at good levels.
These are about 4 weeks into flower, about a week ago I noticed leaves were yellowing between the veins throughout the plant. Although I’ve never added nutes to living soil in the past, I panicked a bit and added a bit of cal/mag. I also noticed the soil had settled exposing a bit of root so I brought it back up with a soil amendment. I wondered if it could be light burn although it was at manufacturer’s suggested height, I raised it about 6 inches. Doesn’t seem to be improving. There’s another plant (White Widow) growing along side this one with same soil and setup. That one is thriving. I’m starting to worry about this one. Any suggestions?

[Stevo60]

Yellowing can be a number of things. My thoughts would be maybe to much watering or lacking nutrients. Hope they come good 😊 here's some pics of bubble gum outside

[User420111]

I'm no expert by any means but I've been gardening a long, long time. Generally, when a plant experiences nute burn, you also notice other characteristics of the leaves. They will canoe, up or down. The tips will curl. They will shrivel, or they will turn brown all over. In my experiences when a plant is nutrient deficient there is far less physical change to the leaf structure. Judging by your photos they seem to fit the latter category better.

Not knowing anything about your setup other than you gave them cal/mag which should in no way be causing nutrient burn unless you just drizzled epsom salt all over the soil, I'd say review your nutrients again. If this is some form of nutrient burn, I'd guess it's excess nitrogen. The plants want lots of potassium and phosphorus now. They have little use of nitrogen so far into flowering stage. So again, review the nutrients and the worst case is your soil is "hot". Which tends to happen sometimes when you buy potting soil with nutrients added like Scott's w/ Miracle Grow... or you'e intentionally added far too much slow-release stuff. People often make the mistake of adding slow release nitrogen and by the time the plant has gotten to the stage where it doesn't need it anymore the slow-release stuff is still slowly releasing. If this is the case, you're pretty much screwed. Try and take your buds to harvest as best you can by flushing with lots of distilled water but don't overdo it and drown them either.

Just some ideas. Take them with a grain of salt.

[Sadpygmy]

I’m using organic living soil. All nutrients are in the soil the only time any additional nutes may be added is when amending with more soil. I have used this method a half dozen times with great success. (Including this strain). This is why I was confused that this was happening. It looked like magnesium deficiency so I added some cal/mag. Watering is a regular schedule with good ph. Everything is within acceptable range, ph, temp, humidity and watering schedule. I just don’t understand it. Looks like this is one of those learning experiences that come along.. We’ll see how it progresses, I’ll try to update how it goes