DON'T YOU JUST HATE IT???
 

I chased a convoy for over a couple of hours and finally manoeuvred into the best position. I lined up two large merchants. Fired two torpedoes at each four hits both ships went down. Only one registered that it went down so instead of 20,000 tons it was 10,000!!!! Watched it sank with my own eyes but it never happened.

You weren't in the North Sea at the time, were you? The waters off the East Anglia coast are bad for this, in my occasional experience at least - so shallow that large merchants etc don't have room to fully submerge and thus (sometimes) are not treated as "sunk" by the game. Grrrr, etc. 🤦🏻‍♂️

Can't remember.