You don't mention what version of PO you are using. For PO the title from the main form that has the date is preferred. Something like Purchase Orders for Adagio 8.1 (09-Apr-2009)

We have seen this happening when there are two screens and the "Document Detail Line" form is moved from one screen to the other, the finder is resized and closed and then the "Document Detail line" formed is moved back. You can try to fix this by moving the "Document Detail Line" to the left screen , the finder should be properly anchored and you should be able to see the item information.