Im currently growing 4 clones of different strains and this one has been struggling a bit, I was a little late to start feeding so it ended up loses a few leaves but it's made a nice recovery, just now this is happening...
I have it in ffof with added perlite and dolomite lime, I had been watering with a little bit of calmag (I use RoDI water), until last watering where I added 2 teaspoons of growbig per gallon. I had bought a decent ph meter but it never worked and I havent gotten around to getting a new one so for now i cant really test the water I'm feeding with.
Calcium and magnesium excess can lockout potassium, but so can out of range pH of the medium. At this point you'd probably be just guessing the cause. Good luck.