Are you using a summary field in the sub-summary part?
And then make sure that you have the correct "sorted by" field and then be sure that your records are sorted in a sort order that includes the sorted by field.
I guess the obvious totaling of each invoices Qty within the sub-summary didn't occur to me - for some reason I thought the summary field had to be the very last total field.
I had a summary field for the total of all invoices, which was working correctly and giving me a grand total in the Trailing Grand Summary part.
All I had to do was copy the same field into the sub-summary part and voila!- thanks!
I did make sure I had the item sorted.