You are over watering. There could be root rot due to the over watering. Let the plants’ pots dry out while 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 switching to manually watering for a bit. This can also be caused by an excessive salt build up (although this is not usually the case). Try going without nutrients for a day or two to alleviate this.