Discount Available Not Populating With Added Line During Voucher Match


I am looking for a solution or possible setup/processing option as to why when a extra line is added on a voucher the system does not calculate discount available. We are experiencing during 3-way match that if an extra line is added for freight, packaging charges, etc. that the discount available does not populate even when the terms code is the same as the purchase order/match line. When you click on the field it brings up a calculator as if you are to calculate the discount and not the system. Any suggestions?