One thing I've noticed it that you have to be very careful about how you aim your torpedoes, especially if you're tooling along at flank speed!
As long as the target is reasonably close to 90 AoB and its bearing is within +/- 30 degrees, there doesn't seem to be a problem with aiming, no matter how fast you're going. But when the AoB starts getting past that, and especially when its bearing is greater than 40 degrees, the problem is that there is a delay, be it ever so small, between the time you press fire and the torpedo actually launches.
Because of that small unavoidable delay, it plays havoc with your firing solution, which is locked based on the moment you pressed fire, and
not when the torpedo actually hits the water. When you're running at flank speed (which is over 38 knots, even in rough weather) you cover a lot of ground during that small delay, and that seems enough to have your torpedo just miss.
Slowing up before launching helps a great deal for those 'off-AoB, off-bore' shots. Unfortunately, slowing up makes you an easier target.
It seems your best bet is to, whenever possible, make your runs in at high speed, as square to your target's course as possible, with the AoB also as square as practical.