We have some Vendor Bills getting created that have no association to the original PO, so the PO still shows as Pending Billing when, in fact, the Bill has already been created and sometimes paid. 

The A/P clerk swears she always gets to the Bills by going to the "Purchase Order to Bill" Reminder on the homepage.  She then filters by Vendor, then checkmarks the Bill she is looking for and hits the Submit button.  She then updates a few fields on the Bill and Saves it.  In most cases, the association with the PO stays in tact and the PO status changes to Fully Billed.  In some cases, the Bill loses it's association with the PO and PO status stays at Pending Billing.