Hey Man... I grew in pure Coco for many years with my AK47 and adding Cal/Mag+ was a must. Using Pure Coir you really have to read up on threads that concentrate on how to use it. Not only do U have to add the calmag but it also stores nutrients unlike a regular soil would.
I found I had to supplement it with either soil amendments or foods. One of the best things I ever did was add Worm Castings and Peat..
Your plant actually looks pretty good.. I know with LED lighting some tend to stay short and bushy like yours is, but it could also start going thru it's big stretch in the next 2 wks. Nitrogen is what is called a mobile element and the plant will steal it from lower leafs if not getting what it needs in the coco.. Add some worm castings to the top layer then water it in.. but don't panic to much bro, she's looking pretty nice ok.. cheers