Recheck permissions on moodledata/temp folder.
Check apache error logs.
Check the moodledata folder temp/backup.
In that folder one might find 0 byte files named with hash. Something like: 18b794c41782d9d6e0ce13fe69f78762.log
Those are indications of successful backups. There may not be any of those files.
If there are such files with any byte size, those are failed backups. Since they are text, open with text editor. They might say something about Moodle not being able to find a handler for X. That will give you a clue.
Also, there might be folder with a hash name like:
18b794c41782d9d6e0ce13fe69f78762
These are the temporary folders created by Moodle for building .mbz backups (.mbz backups are really .zips). Explore those. To see humanly understandable file names one will have to cross reference the files.xml file with the contents of the files directory found in the temp archive.
If one finds what appears to be completely constructed folders or a .mbz file still there, it might be an issue related to OS/php not being able to move/copy a large backup to the other designated area.
Those temporary files/folders in backup can be removed manually (safely).
If one finds nothing that stands out, consider turning on debugging and run a course backup manually.
In http://docs.moodle.org/22/en/Automated_course_backup
there is a note:
Note: Automated course backups automatically skip courses which are unavailable to students and have not been changed in the last month.
Have you tried the automated_backup.php script in admin/cli/ yet? That doesn't involve any cron and restricts action to just that process.
'spirit of sharing'. Ken