Sales tax and Misc Charges remaining on PO after vouchering

Situation:  Puchase Order is set up using a taxable vendor.  Tax is calculated on the Purchase Order and is showing correctly on the PO line by ship to.  Receipts are processed for all lines of the purchase order in full.  Vouchering using the copy receipts function and entering the tax in the sales tax field.  Matching process is run successfully.  PO won't close without being force closed.  I am suspecting that this has something to do with the tax or misc charges.  Why won't the PO close?  Does the tax actually get received behind the scenes?