I honestly don't see how a plant can die, unless you forget to water it. Or get some fucked up disease that wipes out your roots or something..
On the other hand, i find it hard sometimes to break seed. Had a problem with barney's farm seeds a while back, while other breeders seeds popped fine. Breeders fault?
Did you charge the coco with calmag? Or are you using pre charged coco?
Usually I wait until third set of true leaves visible, but some strains show need for nutes before that. If you do feed them, you might want to start with 1/4 strength nutes.
You didn't answer my question about the calmag. I can help you but I need to know the conditions.
Did you rinse the coco well? Are you feeding her? What are you feeding her? Let's start there...