Received items not visible in Item Register which is due to Item not marked as Inventory hence it’s not counted as stock.
1. Open the PO> Undo the Receive.The PO will change to OPEN status now.
2 In receiving register> right click the item in detail view screen> click jump to item details> goto the settings tab> tick the inventoried checkbox> save the settings.
3. Once complete then do the Receiving back.