The main reason you'll miss is because you have either miscalculated AoB or speed. Also if you don't open torpedo doors before you fire then the torpedoes will be delayed often long enough to miss astern. And there is a stock bug where the speed of steam torpedoes is set to slow when its at fast and you need to manually set torpedo speed everytime.
I suggest continuing practise with the torpedo mission until you don't miss very often.
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