Hard to tell without pix, but I would guess overnute, ph, or roots not being allowed to dry between waterings. (ferts + MG Moisture Control = possible overnute) But shitloads of potash in the nutes could be bringing the ph too high. Might be overwatering. (they call it moisture control for a reason, lol. It retains moisture)
Filling-out the troubleshooting form would help narrow the possibilities.