Please check the following :-
1. Check if the Notifications are getting created , you can check it from the Notification Worklist or from WF_NOTIFICATIONS table
2. Even the Workflow status diagram will let you know whether Notifications were created or not.
2. If the Notification got created but users did not receive an email, then it can be due to one of the following reasons.
a. Notification Mailer is down.
b. Notification Mailer is pointing to a wrong server\configuration issue.
c. Notification mailer is up and the server configuration is correct, but an Override address has been set by DBA to restrict emails going - to all the users.
If you can see the Notifications in the worklist or in the WF_NOTIFICATIONS table and for any of a,b,c related as mentioned above , then the DBA needs to resolve it.
3. Check Oracle Support (support.oracle.com) for your workflow type if there are similar issue logged already sometimes it could be a patch issue as well.