Have you done a slurry test of the medium to get a gauge on the pH of the root zone? Sorry if you've been through it all, I just couldn't be bothered going back now haha.
If you put a slice in the fabric pot, steal a bit of medium near the thick of the root zone and mix it with distilled/RO water, it'll either move to acidic or alkaline. If it drops rapidly your too acidic, and if it rises alkaline at all you need to shift down.
Sometimes the runoff isn't completely accurate and a slurry test is the best bet to get a good grasp on the root zone environment.