Unable to Send WF notifications for AP Invoice approval workflow - R12

Hi,

This is on r12.1.3.  The Invoice approval workflow is not sending notification emails. We have tried all we could but email notifications are simply not going out.

Steps executed.

1. Make sure email addresses are valid
2. Make sure all email preferences are set to receive emails
3. WF_NOTIFICATION_OUT was cleared and rebuilt two times
4. Ran WFSTAT.sql for complete diagnostics but shows nothing.

Despite of all this we are not able to receive emails. We need to send notification emails when an Invoice in AP is in "Initiated Approval" state.  The strange thing is that this issue is only with payable invoice approval workflow. All other workflows are receiving emails just fine.