If the pictures are always underexposed by the same amount no mater what the subject distance, then the camera/flash pairing may not be accurate. try over exposing the picture by opening up the aperature of the camera by 1/2 or 1 stop than what the flash manufacturer recomends if you are using the auto or manual version.
If yours is the TTL model, then the flash should have a ev control if not, you can also try setting the EV on the cam to +1/2 or +1 until the pictures expose correctly.
Of course, do note that your camera guide number is 24, so if your aperture setting is at 2.8 and iso is 100, then you should not try to take anything beyond 8 1/2 meters, half that if you use iso 50 and even less if your aperture is smaller. Hope this helps.