[propbeanie]

Are they actually hitting the target ship, giving you a "Thung!" sound, and then a crewman reporting "The torpedo was a dud, sir!"? Or are you just not getting an explosion, and assuming the torpedo is a dud? What can happen if you do certain modding, the torpedoes may well be passing through the ship, and not exploding, due to a "collision detection" not being triggered in the game. Therefore, it might look like a dud. Now, also, the Germans had issues with magnetic detonators and torpedoes running deep, just like the USN had to deal with. It didn't take them quite as long to fix their issue as it took the USN, but there was still a delay in that. What you could try is to untick the "Dud Torpedoes" in the game play settings when you are in the office, before you go on patrol. If you do single missions, you can use the Settings choice on the main menu. Either way, you have to diagnose what the root cause of the issue is first, before you can fix it.