by Emma Richardson.
Flushing the backup_controllers table does not affect existing backups. It basically has the effect of resetting your backup status to a fresh start. So, it should have the effect of at least getting them all to start once.
If you have are having an unfinished error, I would check that your timeouts are extended in php.ini. They often default to 30 - I believe that I have changed mine to 360. Backups often take some time to complete.