SAP archive log backup not running on schedule

Frequncy backup works like this - the scheduler checks all policies every 10 minutes.

When it finds an open window, it checks the frequency - 1 hour in your case.
It now checks the image database - did a successful backup for this schedule run in the last hour?
If the answer is no, the backup will be submitted.

You can try to 'kick-start' it by manually starting a backup exactly on the top of the hour. As long as resources are available, backups will now run every hour on the hour.

The problem comes in when all resources are busy and backup is queued.
If backup only goes active after 15 minutes, the new time for hourly backups will be moved accordingly....

If you REALLY want backups to run the way you want it, you need an external scheduler or else get the OS scheduler on the client to start the backup script as per required schedule and remove the Automatic schedule from the policy.