adobe acrobat calculation fix

Hi, I have a pdf file already created but have one issue. we have two columns that sum up input totals. These work fine. But we have a third field that is suppose to show the difference of the two columns if subtracted but that is not working. Just a blank field. Thats all that needs to be fixed. i will have more PDF work in the future so im hoping on finding someone i can count on for the future Thank you