The Mark-6 magnetic feature was, it turned out, based on a pair of false premises. First, that the earth's magnetic field was essentially the same everywhere and, second, that a steel-hulled ship is going to disturb that field.
In fact, the earth's magnetic field varies considerably. An exploder that worked unfailingly off Newport could fail miserably in the Pacific. And it's a relatively simple process to degause (demagnetize) a ship's hull—something that was done routinely to warships and others going into combat areas once magnetic mines were introduced.
The American problem was compounded by RAdm Robert English, at Pearl Harbor, and RAdm Ralph Christie, in Australia. Christie had worked on the Mark-6 exploder at Newport, and was convinced that it worked. He presumed that any problems came from poor maintenance or other user error. And it wasn't until English died in a California plane crash, and Lockwood took over at Pearl, that anyone would really listen to the commanders. Lockwood allowed the magnetic exploders to be deactivated on Pearl Harbor boats, though Christie persisted in mandating their use for a while longer.
But there was a third part of the problem. Because the captains had been under orders to use the magnetic exploder, and had been setting their torpedoes to run the required five feet under their targets, few of them had had the opportunity to realize that that contact exploder was also defective.
Now, time after time, a perfect shot would send a torpedo squarely into the side of a target, only to have it fail to explode. It might punch a hole in the side of a freighter, but most likely not something that couldn't be repaired at sea. And with a warship, made of thicker steel, it might do nothing more than cause a small dent.
Curiously, bad shots, made at extreme angles, where the torpedo hit the target at an oblique angle instead of square, very often resulted in the warhead detonating and the target going down.