I grow in
Promix and have for decades, but I haven't seen any issues like this. If you add
cal-mag to
Promix, which is already buffered, then it's possible that your Ph is rising and causing a deficiency/lockout. The common denominator seems to be the Emerald Forest, but I'm pretty un-knowing when it comes to that brand of growing medium or nutrients. A foliar application of magnesium would bypass the roots and show rapid improvement if the issue is a magnesium lockout, although
Cal-Mag can cause issues with magnesium getting locked out if used excessivly.
Cal-Mag causing magnesium deficiencies? Yep... if the Calcium raises the Ph too much, Magnesium is one of the 1st nutrients to be locked out, from my experience... but this is more complex than a simple deficiency.