i am pretty sure that the database structure is a little different on your install. Did a importer for an phpbb clone (CBACK). Many field were slightly different like (userID and user_id … ) guess your importer needs also modification.
The included importer is very well writen, documented, and never would map all posts to admin user if db fields would match. What i can tell you from a 1:1 import of a hughe 6gb DB ( avatars, user fields, user custom fields, user settings, private messages ). This is a bunch of work to get it running ( if you need all of your data ). If not, easy one, few hours work.