Pictures?

In straight coco, I feed NPK as soon as I see true leaves.

Yes, start as soon as you leaves but start feeding weakly such as 10% or even less.. You did rinse or soak the salt out of the coco??

Coco is an inert media. Inert means that it has so nutrients in the media at all. Completely sterile environment. Think of it like putting a baby in a hospital room. You would need to bring the baby everything to the room. There is no food in the room for them to eat. If you don’t they will get hungry. Same exact concept with your plants and the coco you put them in. A very wonderful healthy environment but now you are their provider for everything

Also if you don’t use canna coco bricks you will need to wash and buffer your coco before using it with plants. Canna is a brand. These bricks are a bit pricey but they are washed with clean water to get all salts out and charged up with cal mag in the water used then rinsed out finally after it drys with just water. Every other brand no matter who they are will say they do this but don’t do it to the point where it’s good enough to put your plants into without experiencing problems.

I’d your coco doesn’t have a charge to it with cal mag it will begin to form bonds with the nutrients you are wanting to feed your plants. This might sound good but it’ll lock out the nutrients from uptake once bonded to the coco. Also if there isn’t enough nutrients in the water or around enough because of dry back your coco will begin to strip your plants of their nutrients to make these bonds happen.