Resending Failed Workflow Notifications :

Check the status of WF Mailer :

SQL> select component_status
    from apps.fnd_svc_components
   where component_id =
        (select component_id
           from apps.fnd_svc_components
          where component_name = 'Workflow Notification Mailer');
         

Find the Failed/Errored Notifications :
 

SQL> select * from wf_notifications
      where 1=1
         and trunc(begin_date) <= trunc(sysdate-2)
         and mail_status = 'FAILED' -- or 'ERRORED'
         and status = 'OPEN'
         order by trunc(begin_date) desc;
         
We can Either Run the concurrent program 'Resend Failed/Error Workflow Notifications' from 'System  Administrator' responsibility.
    > parameters are optional, please select the parameter values accordingly so that only required notifications will get processed.

     Mail Status                         : Error/Failed
     Message Type                    : Workflow Item Types
     Recipient Role                   : Recipient UserName
     Notifications sent on or after     : Date
     Notifications sent on or before    : Date
     
     or

We can Re-send the Notifications from WF Status Monitor page :
>> Goto Workflow > Status Monitor
        >> Search with 'Type Internal Name' and 'Item Key'
             >> You can get it from wf_notifications table

        >> Click on 'Activity History'> Select the Activity with 'Notified' Status and 'Rewind'
            >> Again select the same activity and 'Apply'
             >> You will get a message for Rewind.

 Check the wf_notifications table for same 'message_type' and 'item_type', there should be two entry with previous one 'Closed'.
 
You may also check if the Notification Preference for the specific user is Disabled, if yes we need to set it accordingl to MAILHTML, MAILATTH etc