There are certain scenarios where Ottomatik is not able to catch a failed backup. Keep in mind these are very far edge cases but none the less we wanted to provide insight we've gained from helping customers debug them.
Sometimes your backup database user will have permission on your the schema that you are backing up, but for some reason not all the tables therein. This can cause those tables to be missing in the backup but overall have a successful status. Just to be sure you should make sure your user has permissions on YOUR_SCHEMA_NAME.* unless you have specific security requirements.
We've had at least one customer report a situation where some views in the database pointed to columns that no longer existed. As a result, the backup was dying mid-process when it reached the view. This caused a backup that was missing all tables that were alphabetically after the bad view in question.