This pops up from time to time.
It used to be you had to actually witness the sinking (through periscope or while surfaced) to get any credit
If you use the external cam you will see that the ship is actually resting on the bottom. I dont know how the game calculates how you get credit for a sinking but occassionally you get jipped.
Ive been denied credit for ships in much much deeper water. Ive also gotten credit for ships that I never even attacked
