hey there , you're right about the slow growth (sorry growmie!!) in coco you really need three things at all times : NPK , cal mag and additives. additives like help break down the nutrients, so the inert medium means there are no bacteria to naturally break down the nutrients so the plant cannot absorb them quick enough. What you need to do is add humic acid , beneficial bacteria (bacilus subtilis, or thuriengiensis) or enzymes every watering ... if you do this you will see the plant blow up. Hope this helps ! 🚀