auth.log is empty (Ubuntu)

When I had the same problem on Debian, I found I had to restart rsyslogd:

/etc/init.d/rsyslog restart

(Your syslogd program may vary.)

The it started writing to /var/log/auth.log again.

Perhaps it had stopped logging after a disk full event, I'm not sure.

See also: https://bugs.launchpad.net/ubuntu/+source/rsyslog/+bug/1059854/comments/9

Check your permissions on /var/log/auth.log