You may have a Ca deficiency. If you are using RO or distilled water or "soft" tap water, you will usually have to supplement Ca & Mg. Ca def will show as burnt or twisted leaf tips, brittle petioles & stems, and yellow/brown spotting on the leaf.
I think you have an issue with lockout to to imbalance, toxic buildup of salts, pH issues or other environmental factors.
I would use a supplement like Botinicare's "cal-mag plus". Add it to the water before you add the MG.