Pretty normal, yes. German early war torpedoes suffered many problems which were mostly corrected as the war went on. American torpedoes had many similar issues as well.

I'm trying a campaign at 100 % realism and no torpedo failed so far. I set depth between 1 and 3 meters impact trigger. 2nd patrol .

Zombits, yeh historically one of the faults was the magnetic exploder being too delicate even waves above crashing together could set it off, plus after the Norwegian campaign they found a fault in an intake pressure feed line which caused a slow build up of pressure in the fin control department at the rear of the torpedo resulting in the torp running sometimes as deep as 16 feet under the depth set, i think back in 2003 they wrote the code for it and i think they did a stand up job....Happy Hunting....SNAFU