Make sure that List View--not Table or Form view is selected.
Make sure that you have sorted your records by the order specified for your sub summary part.
Make sure no part of the label text's selection handles touch or cross a part boundary. (Looks to me like this is not a problem here, but if all else fails, click on the text while in layout mode and check.)
Yep, that was the dumb mistake. It works in List View. I was looking at it in Table view.