Hello Spunky and welcome aboard!
What your problem is called is Angle of Bow or AoB. You need to hit the ship at pretty much a 90 degree angle in order for the pistol of your torpedo to impact correctly and detonate. So you have to be perpendicular to your target and line up a 90 degree AoB shot...unless you use magnetic torpedo pistol which you must set at a deeper depth than the ships draft. I hope this helped!