PO Closing when not fully matched

 We have PO's that are being closed prematurely.  The latest one is a one line PO for $15,000 with two schedule lines; the first for $5,000 and the second for $10,000. A voucher was matched, processed and paid for $5,000 and the schedule shows as fully matched. A voucher was trying to be entered for the remaining $10,000 but it was discovered that the PO was in the status of closed/completed. This schedule shows as unmatched and the header status when queried on the table is Partially Matched.  Our close date is set for 60 days and tolerance set at 5%.