I'm probably the absolute last person anyone should ask about Poser technical problems.. Well, aside from a few very dead people from the 1600's.  However, I've known simple "scaling" to be a culprit sometimes.  If you scaled the head/neck to something other than 100%, try creating a morph for that, dial back up to 100, enable the morph and then try to export again.

Like I said, it's dangerous to follow my advice and could result in the destabilization of several third-world countries.  But, it's all about how desperate you are, isn't it?  ;)