PO Email Notifications - link to 'Approve'

We have approval notifications being sent for Purchase Requisitions and Purchase Orders. The Purchase Requisition emails have the 'Approve' 'Reject' links etc at the bottom of the email which work well. We want the same functionality for Purchase Orders. We used to, before moving to OMCS but now the links are no longer appearing. We only have one Workflow Notification Mailer configured, so how are the 2 notifications different ?