Follow this procedure if site backup is not functioning as you expect.
How to troubleshoot site backup
Check for error or warning messages from the SMS Backup service (SMS_SITE_BACKUP).
Look in the Event Viewer for account and access violations.
SMS_SITE_BACKUP runs under the local system account. Verify that this account can access any remote locations specified in the SMS Backup control file, and that this account has the privileges to perform the tasks in the SMS Backup control file's [Tasks] section.
If the SMS_SITE_BACKUP log file is enabled, look in the log file for SMS_SITE_BACKUP on the site server: <Configuration Manager installation directory>\Logs\smsbkup.log.