Is the jaw connected to the head and the neck or just the head? Poser will not allow the former. Are all your polygons in the groups that you think they are? No stray ones in the neck that think they are a head? If your head is facing in the same direction as the standard Poser figures then the gimbel order for the jaw should be zyx (or zxy but z must be first). UV mapping will have no effect on joint performance.

Hang in there we have all been through it. If all else fails you are welcome to send me the files and I can advise you from there. Won't do it for you, I know that's not what you want but will do it with you.