Depending on the users SysSetup for when Change Order Approvals is required, sometimes the PO's appear to get stuck open. But is it due to the fact that your change order settings are not forcing the refresh of the order status.

To be able to close those PO manually, you need to modify either the price or qty to some number and then without saving change it back to what ever the original number was. Now save the record. this will trigger the change of status to become "EDITING" and need of approval. When "Approval" button is clicked the PO status will become "Closed"