When you try to edit a merge field or other data field in a table cell, you may be unable to edit the field because you are unable to read the entire field code. Or, the field code may fill the cell vertically, therefore making it very difficult to edit.
This behavior occurs when the table cell is too narrow to view the entire field code, and when you press SHIFT+F9 to edit the field code.
To work around this issue, follow these steps:
Right-click the field code that you want to edit, and then click Edit Field.
In the Field dialog box, click Field Codes.
In the Field codes box, make the changes that you want to the merge field.