If you add a field to the form, or remove a field, you should also update the Use property of the field. This property indicates whether the field is 'in use', or not.
Only fields that are considered to be in use are shown in lists where you select a field (for example when creating a report, a sort order, or when searching). Use the Auto Update Use Property function to automatically update the Use property of all fields in the table.
Choose Auto Update Use Property from the File menu. Confirm by clicking Yes.