Cmathias, the work around that we have used is to add a dummy item to the PO  that never gets received.  The reason the PO's do not show up is that they are closing automatically when they are fully received.  Our dummy item is call Test - Do Not Receive, Qty 1, Cost =0.00.  This ensures the PO remains open and is available to attach bills to.  We then have to manually close all completed PO's during our month end review of open PO's.  It's a kludge, but it wass the only way we could find to keep the PO's available to attach bills to.