I agree, it is either manganese or potassium. My vote would be to use a regular NPK fertilizer, I have even used some of my veg fert for one feeding, since the flowering ferts are lower in nitrogen. If that does not work, I would say you have it down to manganese. I just had leaves like this, and did a good NPK feeding like I said, and it cleared most of the issues up. I am sure you are keeping your PH in check, right? So it would not be your PH locking nutes out? Wishing you luck, man! Love all these opportunities to learn with my buddies!