Geezus I see this same problem everywhere. There is no escaping it, we are all doomed. 8 grows always the same issue. I've never had a perfect grow I've changed everything and always day 25-45 I get the same problem. This grow I think I have fixed it. I changed two things. One I changed ferts and two I have stopped pH'ing my water. I am in promix also and what I believe you are seeing is an imbalance in your promix because your are pH'ing your water. Promix is peet and lime it is setup to give you a 6.0-6.5 pH in the soil. When you add pH adjusters to the water your creating an imbalance in you media. Get a soil pH tester test pH of the media. I will bet your pH of the soil is 6.9-7.2. this is too high. Start feeding without pH'ing the water and after 3-4 waterings this will start to stabilize and the media pH will start to go down. It's either that or the nutes I was using were just crap. Anyways your setup is much more elaborate then mine so you probably have more experiences then me, but figured I would chime in and say my two cents.