After adding a new field to an incremental table, that field remains entirely or partially NULL.
Incremental tables update only new or changed records by design. Without additional intervention, the new field will only be populated for records created or modified after the field was added.
The easiest and safest way to address this is to do a full load of the incremental table with the new field. However, that can sometimes be time consuming if the table is very large.
In some cases, the field can be updated manually. If the new field is a custom field with a transformation, the transformation can be applied directly to the table manually with an UPDATE command.
If the new field is a lookup, it's possible to copy out the section of the Data Cleansing Script that performs the lookup and run it manually as well. Since the script to applies the lookup to the raw table, you will need to make a minor modification to apply the update to the valid table instead.
If the new field comes directly from the data source, your options may be more limited. If the data source and the TX DWA database are located on the same server, it may be possible to join the valid TX DWA table and the source table together to perform an update. If the two databases are not co-located, you may have to do a full load of the table to populate the field.