Do you know the pH of your substrate? Maybe you didn't wash the coco you're using well or you simply didn't wash it at all. One thing I've learned is to always measure the pH of the water I'm going to use, no matter its source: reverse osmosis water often has a very high pH, the same goes for rainwater (I've measured it and found surprises like a pH over 8). I would wash the medium with water at the appropriate pH level, ideally 6.5, but never higher than 7.0. You can lower it with solutions they sell (pH down) or you can use a few drops of apple cider vinegar as a DIY method.