OP, as you see there is a lot of focus on environment in the answers here. And magnesium and also calclium. With leds its quite hard to get the plants to transpire enough:
You need to dial in temps and humidity according to vpd: vapour pressure deficit, and observe what vpd is good for each stage of growth. Its not a 100% science; you also need to be able to read your plants: when they show the same kind of plant posture as you showed as your healthiest growth check what vpd your environment gives you and use that as your baseline. This is the way to drive healthy transpiration which results in good nutrient uptake. Vpdchart.com is your friend in this. Remember that leaves under leds are often several degrees colder than environment; all the lights you mentioned keep the leaves hotter than the environment which throws the numbers off a bit so you can never expect the same behaviour from led grown plants as hid grown. Vpdchart lets you adjust those values for convenience.
Once you start getting this right and increase light intensity then you also need to think about temperature floor: since the plant is freezing a bit the metabolism cant really keep up with the amount of light your giving: for full intensity flower temps need to be over 80F and your humidity needs to be adjusted to the right vod for the stage. +80F should be your temp floor for full intensity but many get away with a bit less in veg under low light intensity: keeping rh high in 80F is very hard so if you see good growth in high 70ies its allright while light intensity is low.
If you still have problems then adjust the nutrients; higher mag seems to be helpfull as long as it doesnt go overboard. But mag have specific ph needs to be properly available so some get better results keeping ph towards the higher side of the ph window.
"Its the light" argument: yes and no; its the light and your failure to adapt to it.
"But ive seen other people having my problem" : if you listen to these guys you're listening to the people who failed to get this working, obviously thats not going to be helpfull. Listen to someone who made it work instead. Growing with leds in the cold isnt easy: the one ive seen best able to do so is
@Moflow , hes already on this thread. I think he uses a meat mat and some magsulfate/epson salt in the pot during veg and grows well were most would fail.
Substrate: the all nutrients in the soil/pot is usually not the best way when youre trying to work this out. Coco hydro is usually the go to solution, but make sure to prebuffer the coco lightly with calmag. Its easier to adjust and control everything that way. Never used promix myself, maybe someone else can talk about how it affects ph. Remember you want somewhat high ph. But not crazy high. Theres charts around for mag availability according to ph. Again this is your friend.
Another cheat code for increased transpiration is using more blue in the spectrum: blue (or even better violet N-uva, or any blue under 440nm) and heat drives transpiration up.
Also make sure you have a temp/rh meter that stores the values so you can check what happens at night etc.
Dont get desperate, listen to what people that made this work is telling you. You have some really good tent growers responding to you here on the thread. This grow might be frustrating, once the plants started going light stunted its kinda hard to get them back on track and to man up to higher light intensities. Next time make sure you do not push to hard at once with light intensity, and if you increase intensity you need to up temps a bit aswell in the same time.
