Does the total appear (1) by itself at the top of a page after all of the details, (2) at the top of the last page of the invoice with details for this invoice below it, or (3) at the top of the first page of the next invoice.
Do you have New Page After set on anything above the Invoice group footer where your total is? Do you have "Keep Group Together" turned on for any group? Do you have anything in a section below the Invoice group footer? If so, does it appear correctly on your report?
-Dell