Update source document in standard purchase order

1. We created requisition from a catalogue and it did create a standard purchase order.

2. After PO has been created, the catalogue has been updated and we need to update the price.

3.  We tried to update price by clicking on the Document and the Document Line in the standard PO so that the price will be refreshed. The problem we are encountering is the document shows the BPA but when click on Document Line, it does not show any lines. We have tried it before and it works but not anymore.