System notification for PO Approval - intermittent issue

Hello,

We are on JDE v9.1 and encounter intermittent issue with the system notification triggered by the purchase approval route. Sometimes, for the same approver and same originator, the approver does not receive any system notification for PO pending approval review. Yet on the order raised by the same originator which routes to the same approver, the notification worked. Its very strange. I have checked the employee setup, employee mailbox, approval setup to the correct employee, and i do not see anything wrongly setup.

Looking for any sharing where if other users may have encountered similar issue, what else can i try to check? P98800 seems to only work for sales workflow, i cannot find any record for purchase workflow. Thanks for your insights.