A find produces an unsorted found set of records. Sub summary parts are only visible when the records are sorted by the fields specified for the sub summary parts so that they can be properly grouped with the sub summary part.
After performing your find. Sort your records in a sort order that includes the "when sorted by" fields specified for the sub summary parts and they will re-appear. You can even produce different summary reports on the same layout by changing the sort order to group the records differently.