Don't ever let coco dry out completely.
If you aren't using a very high quality coco, it needs to be well-rinsed before use to remove excess salts. That can cause symptoms similar to what you are seeing in extreme cases.
Use coco-specific nutes on the stuff, or at the very least, add calmag plus to your routine even if you aren't using ro water. It scavenges calcium and can cause trouble.