Backups run intermittently

I noticed this morning that my backups have not been running consistently http://imgur.com/RFhsO8s. At one point, it went 11 days without running a backup and then started again. I have plenty of space so it is not that my backup is crashing. Any ideas on how to diagnose this?

Thanks!

So I had to manually run the backup to get it to run. When I did so, it ran a full backup successfully. But it has not run on its own since. Regardless of what the problem is, shouldn’t an administrative email be sent when backups fail?

Roughly 2 days after manually running a full backup, it successfully ran an incremental backup. It has now been more than 3 days since its last incremental backup. Any help here?