Are you in list view or form view? If you are in form view, change to list view.
Are the records you want in your found set? If not, perform a find to get them. (and sort by the order needed for your sub summary parts.)
Did you place the fields from your task records in a "body" layout part or a "sub summary" layout part? If in a sub summary part, you need to change the part to make it a "body" part if you want to see individual task records listed under each report sub head.