wow dude that sucks. I used coco and had great success with it.
First off, you wanna aim for a pH of 5.8...thats the sweet spot. Your plants aren't absorbing any nutrients because the high pH is locking them out. First thing I would do is flush, thats the good thing about coco, you can correct a problem fairly quickly by flushing with pH'd water. I would flush until the run off is at 5.8, then feed.
I usually mix my nutes and pH right before I feed, I dont wait at all. How long are you waiting to feed after you mix?
What kind of nutes and pH down are you using??