Welp, this is common with new comers to coco.
The first two things to take note of when using coco are:
Coco coir has zero nutrients in it, it is completely inert.
Some brands of coco coir contain sea salt which needs to be rinsed out.
Are you fertilizing with anything? All plants grown in coco coir must have fertilizer once the first set of true leaves appear. Synthetic nutrient are immediately available to the plant once they are applied to coco. If you are using organics fertilizers in coco, make sure you mix a microbial innoculant into the coco as well as fertilize the medium 14 days prior to planting into it. When using organics, the microbes must break down the organic matter into a form that the plant can take up, and that takes time. Hope this helps.