If you are using an Invoice, there is a "Class Box" at the top of the Invoice next to the name. You may have added "Class" in a column in the body of the Invoice, through "Customizing your data layout" for the Invoice Template. If so, it will need to be entered twice. One entry at the top of the Invoice and in the one "Class" created in the body of the Invoice. Any reports you run filtering "Class", will now show your different classes, or if none, "uncategorized class".