When you first install the samba package on debian, it does this neat thing for you where it'll generate a samba database file from your existing /etc/passwd. However, if you add more users to your system, the samba database is suddenly out of sync. If you rm /var/lib/samba/passdb.tdb, rm /etc/samba/smbpasswd and then dpkg-reconfigure samba, it will regenerate the samba password file for you.