Purchase Orders are missing from Post Vendor Bill Variances page

We have some POs that are fully billed, and the vendor bill amount does not match the amount on the item receipt or on the PO (the receipt and PO have the same amount), but they do not appear on the Post Vendor Bill Variances page. That page is set to Transaction Type: Purchase Order and other POs do appear, but many do not. What would cause this behavior?

(We did try implementing Match Bill to Receipt a little while back, but it didn't work for us so we aren't using it. But just to test I changed the Transaction Type on the PVBV page to Bill and no transactions appear at all.)