[UKönig]

German torpedoes of the WWII era, at least before 1942, really were hit or miss, more often, miss, in conditions where it should have been impossible to do so.
They were never tested fully and properly between the wars, so the makes and models they went to sea with, weren't much better than at the end of 1918.
The contact pistols could be unreliable if striking the target at angles greater than 45°
The magnetic influence detonator was overly complex and prone to many malfunctions, such as detonating too early, or not at all. Seems that the MID had its own battery. Which caused its own maintenance problems, which was corrected in later production torpedoes by having the detonator draw power from the torpedo battery itself.
So if you fire a torpedo on magnetic, and the MID battery is dead, then you will have essentially a contact pistol, which will not explode, because it was not set at a depth to physically strike the hull.
They also had trouble keeping depth, not so much because of bad trimming or loose mechanics, but because the hydrostatic valve that controlled depth, was inside the mass balance chamber at the rear of the torpedo, which was not airtight or pressure proof. This problem was discovered by Otto Ites of the U94, in Jan of 1942, and was the last major defect to be fixed. Up until then, it was advisable to remove about 2m off every shot in depth, before launching.
Still have a few more variables to consider when lining up a shot, but that should explain things a little.