There might be a bug, but am wondering if the cron job has been run on a regular schedule. There is a column (a unix date/time stamp) in tables related to when courses were last backed up or altered/changed.
Turn on debug.
The automated backups are run according to a schedule in automated backups config. Set the time for something close to your current time. Then run the cron job. In the output of the cron job, one should see some lines related to automated backups. Might have to run the cron job more than once to get automated backups to kick in if one doesn't time it right. Consider running the cron job from the command line and run the /moodle/admin/cli/ cron job.
Since have debug turned on, Moodle might report something useful ... copy and paste the text of the error to a notepad document for later sharing here.
If you are running apache, check apache error_logs. The autobackup process runs under user apache (if using apache) and must have access rights to be able to write to the c:\moodlebackup folder - check permissions/ownerships of that folder.
'spirit of sharing', Ken