Forgive me if you already know this but I feel it is always worth repeating for others who may have the same issue.
In addition to the excellent advice about hitting the "check" mark, are you opening your torpedo bow doors (Q) prior to firing?
I know that I didn't when I first started playing. They did not call me Sub Lieut. SteepLearningCurve for nutton.
If you hit fire without having previously opened the torpedo door, your crew will do so automatically but this will delay the actual launching of your torpedo by a few seconds. This will cause your torpedo to miss behind your target
