If you still have the users in the old ubb format then you can reimport the whole installation into a development database and see if they make it over.
If they were lost then I suspect that they were corrupt or did not match the criteria for viable accounts or one of them was corrupt and that caused the row counting to be off for the remainder of the import.
Did you lose all your Members AFTER a certain point? Meaning 399 - 1099?
Regards,
Brett Harris
Infopop Corporation
