As far as I can figure it can be because of; nutrient def, over watering, heat, light, pests, lockout, I've had them all.
Good luck
I've narrowed out heat, light, and over watering (rdwc)
I don't THINK it's over nutrients, ppm 1100
Was told calmag deficiency. Was using 3ml a gallon instead of 5 cause I am using tap. But when I ran ro I was using 5ml and still same issues.
Using a smaller tent with some t5 lights and a 10gal dwc with room for 10 small plants. Running simple Lucas formula with cal mag ph 5.8. They seem to do fine first few weeks.
Couple weeks after moving to other tent with rdwc system they start looking like this.
At first was doing a diy bucket type aero then dwc and now rdwc and all the same issues. 4x4 tent, mammoth 6 bar led, have turned down, 6in fan with filter. 80 degrees and 52rh.
Was using botanicare calmag, silica, veg, hydroguard and liquid karma. Ditched it thinking just go simple even though I used all that 10+years ago and was fine. General hydroponics Lucas formula and calmag and super diluted southern ag as hydroguard replacement now. I've blamed everything and trying whatever I can.