If your torpedoes are running out of steam (or batteries) before reaching their targets your taking shots from WAY to far out.
In all the time Ive been playing Ive only had one torp come up short of its target and that was because I lined up the wrong column of the convoy
Preamature detonations are another thing. If the torpedo suddenly dissapears from the screen most likely it was a premie. If the sort of slow and stop but remain on the screen ... they ran out of fuel.