Hi ranafaisal,
Have you set up a cron job pointing to your-domain/your-path/wp-cron.php?
This should ran every 5 Min. There is a good tutorial at here.
If this doesn’t help you could try this:
Goto to your phpMyadmin and on the database which is used by wordpress. You can get these informations in the wp_config.php. Then go to wp_options and search for option_name LIKE ‘newsletter_diagnostic%’. Delete these 3 items. Then call the wp-cron.php via your browser. The items should be recreated.
Background: Every time the cronjob runs the option ‘newsletter_diagnostic_cron_data’ is updated. If the mean time between the call are > 400 this message is shown.
@see plugin.php@381.
Hope this helps you.