I would like to copy and paste mesh and (rigify) armature from a blender session to another blender session of even in the same blender 3D window,but part of the armature is not copied and pasted correctly and when I select the root bone to move the copied version,part of the face armature does not follow the rest. Do you know why it happens and how can I fix this behavior ? I have recorded a video to show you. You can find it here. Thanks.
So, I would NEVER use Copy and Paste for a Mesh and Armature combination. I would always duplicate in the same Blender project by selecting BOTH in OBJECT mode and then key SHIFT+D and move the copies.
To move to another Blender project, I would ALWAYS append the mesh and armature in one operation from the first blend file to the second. To append, click Append in the file menu and select mesh object, if you have parented them properly, the armature will come with it, in its entirety.
At around the 1 minute mark in your video there is an error message at the top, look in the console and tell me what that was…