Hello,
You will need to troubleshoot why the cron job in scripts/cron_mail_queue.php isn't working properly. If you installed via the script, the cron job should have been added for you. If you manually installed, then you'll need to add the cron job per https://docs.itflow.org/cron
To troubleshoot the job, you can tell it to output to logger (| logger -t itflow-mailcron) or remove it from cron and manually run the script as www-data.
It sounds as though the job isn't even running, as the attempts should be increasing.