why auid always unset?

Tue Jul 23 22:49:31 UTC 2013

I read my audit logs.I always see lots of auid values are 4294967295. Even
when I delete a file, the value is still 4294967295?
I added pam_loginuid to  gdm, login, kdm, sshd, vsftpd. Howver, it is still
the same value?
I wonder what is wrong?
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