Magnesium availability begins to taper off below 6.0 pH. If you're growing in coco you should be using calmag. Check your pH and make sure it's around 6ish / 6.2. I like 6.2 personally. I feel it's the sweet spot in coco. Coco tends to ride around the pH dial so monitor regularly. For a quick fix add some epsom salts to your next watering. But be sure your pH isn't low first. If low, adjust. If not low, add epsom salts to your watering, or better yet, get a bottle of calmag and start using it regularly. Epsom salts will fix a magnesium issue pretty quickly.
