I think you are right on, I have used grow dots also and had the same experience. Coco + grow dots + auto pots is probably just not a good combo since coco needs almost daily runoff to keep balance. I suggested the same things as you during the grow and no one agreed with me, so at least it’s nice to know I was not alone. Zinc deficiency is also extremely rare and almost always related to other issues so it being the primary problem seems extremely unlikely. As Occam’s razor suggests the solution to most problems is almost always the simplest one.