The hierarchy of the models stack is important. Skin:skin means you have a modifier on the top of of the stack that is not SKIN, to remedy this simply drag whatever it is just below SKIN and try again.
The skin modifier must be on the top of your stack if you have edited the UV mapping at all drag the unwrap UVW modifier between SKIN and your editable mesh
I wrote this on body editing
http://www.lucasforums.com/showthread.php?t=151385) hope it helps.
Step #6 covers this very problem when i came across it.
Good luck
svцsh