PH could be your prob but I don't think so . It's one thing to diagnosis a deficiency, but you also want to know WHY !!! Sometimes K and Mg become unbalanced, especially during flowering, when P and K applications are heavier than Mg. Here's a great article on the subject. https://www.kpluss.com/en-us/our-business-products/agriculture/kali-academy/facts-worth-knowing/Interactions-of-potassium-and-magnesium/