Coco coir is not treated like soil. So with everything I would say it is ph related from the years I have grown in coco. Possibly not enough water or irrigation. Coco works best with a lot of water transfer. It's Hydro in a potter.
Don't use Advanced Nutrients. overpriced product. Look into other options and invest in a EC pen, read a few books on how to grow and form a plan for your next grow.