Salute, Dan !!!
There are a whole bunch of things that go wrong even when we do everything right !!
First thing that comes to mind is your torps may have run under the target. (I usually watch steam torpedoes on their run with the external camera so if I miss I can learn from it.)
If you have Hsie's mod and are using realistic torpedo performance then there is a very good chance that they were duds. He mimics that by making the torps run too deep.
Is it possible you accidently got your AOB backwards? (starboard/port)? Did you remember to put the TDC back in auto after you inputted the settings?
In real life, I think that most misses were due to errors in the target speed. That is what probably causes us to miss as well.
AD