If you're still in Promix HP, start adding nutes - you're fine with very light feedings but make sure you're supplimenting Calmag on top of your light feedings of base nutes. For promix HP, feed/water at 5.8-6 ph. If you call Promix, they'll tell you water PH doesn't matter, the alkalinity of it does. But I have hard water, and it's fine if I ph it.
To my unexperienced eye, it looks like a lockout or a N deficiency.
Check your runoff PH. Should be 6-6.8 in Promix. Anywhere outside of that range is your problem. If you haven't fed and you're still in promix, that's your problem. If you've done everything right, then I have no idea! haha.
Good luck!