Make sure your Store URL (configuration) has trailing "/" https://yourstore.com/
Check the System Log for any error messages.
case 1 1. Store URL (configuration) has trailing "/" and HTTPS 2. Enable SSL 3. No errors in logs All scheduled tasks fail when running automatically but not manually
case 2 1. Store URL (configuration) has trailing "/" and HTTP 2. Enable SSL 3. No errors in logs All scheduled tasks started successfully