Check and see if the items from the PO where actually pulled into the bill.   I have found that sometimes they are manually entered, which keeps the PO open.   Quick way to see.  

Open the bill. Change the Vendor,  move to another field. then change the Vendor back.  It should allow you to re-choose the PO.  Confirm the quantites/items applicable to the bill. (change if needed)   Save.   

I hope this helps.