User cannot see the 'received' amount or units on the Purchase Orders screen without having to go into the detailed transactions. The vendors also want to see the received items if the user revises a PO and re-sends it.
To see the received items on a purchase order, go to Actions > Receive at the top-right of the screen. This will take you to the Items Receivable screen where you can see the expense units or service hours received against each item. Check the Core Help for details.