Hi,
identify the reason is not so easy, do some checks to ensure that the email settings are correct and the component is updated to the latest version.
After that you can try to execute programmatically the URL to send email reports opening in your browser for example:
http://www.yoursite.com/index.php?option=com_jrealtimeanalytics&task=serverstats.emailxls&tmpl=component
Opening this URL in your browser must have the effect to send the report email and show a '1' on the page.
You can even schedule a cronjob with this URL if needed.
