I need pics of the plants mate, one leaf isn't enough for diagnostics, especially in the state it's in,... if all 6 are doing this, then either the nutes or the medium are likely suspects,... any idea what the pH in-pot is? --that's critical info to know, it'll first determine if you have a lock-out issue, or if it's OK, then the finger points back to the nutes or the Promix,.. bad batches happen! Also, just how hot and humid? If the leaves can't cool/transpire, then the flow of things from roots up will be adversely affected, leading to other issues,....