According to the game files, each tug should add +1 to your fleet's max burn. Are you running your fleet under Sustained Burn near or at 20 burn? I'm pretty sure the game caps your normal max burn speed at 20 (excluding hyperspace storm effects), so tugs would be cumulative only up to that point.
(Does Sustained Burn double the +1 bonus from each tug, or does the tug's bonus come after the doubling?)
Also, check what shows up in the hover-over tooltip on your burn speed while travelling. Hit F1 and it should list a breakdown of what's affecting your burn at any given time, including any bonus from tugs.