To make it short, I have the exact same problem as described by the user here: http://wordpress.org/support/topic/problems-inserting-terms-in-a-hierarchical-taxonomy
When I add child terms into one my Parent taxonomy term, the child term magically disappears from the list in the taxonomy admin panel (and doesn't show up on the front-end of my website when I display the list "categories" of my custom taxonomy ).
However, I am using the Types plugin for creating post types and taxonomies.
The problem started when I migrated my test site into the live server.
I updated Types plugin to the newest version, but it still does that.
Does anyone know how to fix this ?