Presumably, you are counting records in this summary report. If there are no records in a particular category, there are no records to group to produce the needed sub summary layout part.
You might want to consider setting up a table with one record for each possible group of records that you are counting. Your summary layout would be based on this new table with the subsummary part shown for these values replaced with the body layout part. You'd then use a relationship match to the current table and a calculation field that uses count to count the records (or an ExecuteSQL calculation field could produce the same count--especially if you need to filter your set of records to count to a subset of all the records in the table.)