Some of the iSuppliers are not receiving the PO Approval notifications from the Workflow process.
Please check the workflow log for more information.
You stated some users of iSuppliers does not recieve mails, any specific user setup which differentiates these users from receiving the PO approval notification. Was this functionality working fine for these user before?
Can you please confirm the Workflow Mailer Service is up and running. In addition, to better assist you troubleshooting please check the workflow mailer log for more information and post them here.
Please also see:
R12: PO Approval Analyzer Diagnostic Script (Doc ID 1525670.1)
How To Diagnose and Troubleshoot Problems In Procurement Workflows Using Scripts (Doc ID 374428.1)
If you can check the log file and/or the diagnostic files and provide us with some information we will be able to better assist you.