Additional Tax Information screen is blank

Hi - we are running 12.1.3.

Occasionally the tax codes we have defaulted onto a purchase order are not as per the invoice from supplier when it arrives. In this case buyer would navigate to manage tax and observe the details in the tax summary screen then click the Additional Tax Information button which takes you to the Additional Tax Information screen. In this screen the tax details such as tax classification and product category may be changed and then click the Apply button and the tax details for the purchase order are amended in accordance with requirements.

On a few cases when buyer goes to the Additional Tax Information screen to make the change everything is blank so no changes are possible. Any idea why this may be please?