Group report based on secondary index file

I'd like to make a group report based on secondary index file which is
indexed by date field.
I've try but it seems doesn't work. The group header which should be a
Date field disappear from form report ( on the preview ) and the report
without any grouping by date.

Note. In the properties ( Table Properties ) , I've set the index name
with the secondary index file.

Could any one help me ?

Thank you for responding.