TI torpedo malfunction

I'm using a VIIB in feb -41 and have armed it with all TI's mainly because of the fast speed option that TII doesn't have. Anyway, the thing I really hate are duds so I have the "No duds" - option checked. However, just on my last patrol I spotted a Granville merchant and shot at it from a good enough angle and when it hit the target there was just a metallic clang and nothing. Same with torp no. 2 and 3. I was using magnetic trigger and depth of about 5-6 meters.