Navigate to Transactions > Demand Planning > Order Items.
Order Items page is not showing some Items with Back Ordered Quantity.
The Inventory Items do not have values on the Reorder Point, or Location Reorder Point. These can be newly created records.
Edit the Item records. There must be a value in Item record's Reorder Point and the Quantity Backordered greater than zero to be able to show the item in Order Items page.