You can reject the PO with response Note field from Notification Summary.
You can see the whatever response Note you have entered is coming in Note column of Action History
Navigation PO Form > Query the PO > Menu > Inquire > View Action History > Note column.
From Back end you can review the rejection note/comment from PO_ACTION_HISTORY (NOTE column) table
Yeah. I had already followed that. The entered comments are not getting stored in the table.
For that you have to give input in note field in Notification summary at the time of rejection.
I think it should work because it is Oracle standard functionality.
I have checked at my end and it's working fine.
Please check again . If it is still not work, Please provide some more details like screen shot or table dump.