When I sort on the sub-summary the date will only display on the header of the first occurrence.
Where is the date field defined? If in a related table, it appears that not all records in your found set link to a record with the expected date. If in the same table as that specified for your layout, then I would check the value of this field in every record as it appears that not all records have this date.
Thank you! You were correct. I have now corrected it and the date shows as expected.