Coco is prone to calcium and Zink defs and if you are not using a coco specific nute than you may be seeing that and it may be causing a lock out rather than anything related to PH. Many coco growers water at PH 6.2/6.4, I water at PH 5.8/5.9 my self. I think that you should get a bottle of CalMag Plus and use it full strength in your res and then half strength the next res change throughout the grow except for the last 2 to 3 weeks and your problems may change.
How often are you flooding?? What kind of nutes are you using?? Bio Biz nutes are generally soil based because the biological bacteria's need to chilate Bad Spelling, in the soil creating the beneficial bacterial environment which makes that particular nute function so well. In an inert medium like coco you are not looking for the same play ground.
Try to dig up some pics on Cal/Zinc defs. Also, are you using coco slabs or loose coco in baskets??
Adieu