If you're using BackupPC, and you're trying to backup a server that is behind a firewall and does not reply to ping, BackupPC will not start the backup, even if you have the SSH port open.
No ping (no ping response from) ...
You can override this behavior and force the backup by selecting the host on the web interface, then going to "Edit Config" and replace the default "PingCmd" command with:
/bin/echo
Check the override box and save. Now the next scheduled backup will run without an issue.