You are over watering. There could be root rot due to your over watering. Let the plant’s pot dry out, checking them regularly. If you buy a moisture meter you can begin to properly manage your moisture, and over time you will not need it. If you place you finger all the way in and you see water on your finger when you pull it out, your media is still too wet. Coir holds a lot of water. On your next round, try to cut your media with Perlite for proper aeration and drainage. If you have an automatic watering system, try manually watering for a bit. This can also be due to a massive salt build up (not usually the case). Go without nutrients for a day or two to alleviate this.