If you are using impact triggers on your torpedos, are you close to 90 AoB? (directly to one side of the target, not ahead or behind?)
Impact torpedos often bounce off the hulls of ships if they strike them a glancing blow.
If you are using magnetic, set the running depth to just below the depth of the keel of the ship (its listed in the page for that shop in the recognition manual) and you don't need to worry about AoB so much
Yes sometimes torpedos fail, but 6 in a row shouldn't fail. Also, if you don't have duds ticked on the realism screen I don't think you get any duds. Any failures are due to rough weather, or bad angle of impact.