1)Check your notification mailer's log file, any errors there?
Here are some example diag methods for you :
http://ermanarslan.blogspot.com.tr/2014/08/ebs-workflow-notification-mailer.htmlhttp://ermanarslan.blogspot.com.tr/2014/08/ebs-122-notification-mailer.html2)Ensure that the users are configured to get email. User mail preference may be "Don't send email" .. (check docs for controlling this.) -- 11i - 12 How To Set Email Style Preference For All Users At Once? (Doc ID 578574.1)
3)Users email address may not be correct (check docs for controlling this.)