Starmark - while you're waiting for an answer to your question, you might want to check out this thread - another pH problem. The thread's kinda long and there's a lot of foolishness in it, but it also has a very good discussion about nutrient lock out, why it happens, how to fix it. It sounds kinda like you're having a similar problem, so maybe it will help you.

Good luck!
PharmaCan Reviewed by PharmaCan on . latewood ,stinky.......anybody.HELP This reaccuring problem has been with me for the last 3 grows,ive been able to finish except for the last grow which killed all plants at about week 5 of flower.no problems seen untill about week 3 or 4 then it comes on quick.White widow grown in rockwool ,ebb and flow,ph i keep at 5.6 however checking runoff for the first time ph was 7 in rockwool ,so i thought i found the problem and flushed a few times from the top of the rockwool and changed rez however problem refuses to be cured.Using gh Rating: 5