Bone tool problem - detaching for no reason

Hello.

Im having a really hard time trying to build my first doll with bone tool… It seems so easy in theory but in practise its so damn problematic… Basically i cant make it work properly… it keeps detaching spine from the head… Here is how it looks like:

I have started with the spine and the head (there is an invisible object above the head to give it its own “bone”), and it worked perfectly before i finished attaching arms and legs :confused:

What have i done wrong?

Thanks in advance for any help.

regards
Falrinth