The Germans suffered a number of problems with their torpedoes. One was the torpedo running deeper than set. I believe they found the cause to be related to the internal pressure of the U-boat itself. Over time, air pressure builds up inside a U-boat, due to the many tiny leaks in the air lines. Over time, a significant amount of escaped air will build up pressure inside a submerged U-boat, and this increased air pressure affected the depth keeping mechanism. This is the reason why the hatches on a submerged U-boat spring open when unlatched. You may see this in the film, Das Boot. One reason why that film is the Holy Grail of submarine films is because it gets 1,001 tiny details correct, unlike that dog's puke of a film known as U-571.
Another problem was with the magnetic pistols, as discussed here. The reason why the Norway campaign brought this to light is because so many magnetic pistols malfunctioned. Later study brought the cause to light: the high content of iron ore in the seabed of the North Sea affected the magnetic pistols.
Between the two causes above, many ships survived which otherwise would have been hit.
The reason why some impact pistols fail to detonate is because they do not actually hit the hull. The feelers at the tip of the torpedo have to strike the hull and be pressed down to detonate. A torpedo striking at a less than optimal angle runs the risk of the feelers not actually hitting the hull. Instead, the rounded portion of the torpedo head hits and the torpedo fails to explode.
__________________
|