I am on QuickBooks Desktop so there may be a variation.  However, when this happens it is usually because a PO has been fully received and an item is later backed out of the item receipt because it wasn't actually received.  

I go to the PO and make sure the item that was backed out of the item receipt is back on the PO

Next, while still on the PO I "Mark As Closed" then "Mark as Open" and hit the save button to the left.

When I go to the "Create Item Receipts" button the PO will magically re-appear.