Alerts not working for a specific user

 Hi Team,

Please note that we have a strange issue with one of the custom alerts defined in standard 'Payables' application.

1. An email notification is sent to PO buyer based on specific conditions due to which an associated invoice is on hold. The invoice will not be released for payment until PO requestor takes some action. 

2. The issue with this alert is that if it fired from a specific user (set up as employee in the system), it fires and shows exceptions.

3. But if the same alert is fired from a scheduling user, it does not show any exceptions even if eligible records exist. This has started happening recently and was working fine previously till about a month back.