Poser has different hierarchy rules than Blender. It appears the jaw is a child of the neck and the head. Poser doesn't do that. A body part in Poser can normally only have one parent. If the head had a row or two of polygons that when all the way to the neck and the jaw only touched the head, it's parent it would work. Sibling body parts can not touch.

P.S. There are hacks to get around this but that's not recommended.