What do you get if you extend the vertical line in the subsummary part several pixels into the parts above and below it?
Does that make a difference?
Have you tried making it a single vertical line running from top to bottom of the layout, i.e. across all layout parts, instead of a separate line on each part?
I tried at first, but the lines can only print on one layout part each. Seems to be whatever part is at the top. Thanks for the suggestion though.