If you didn't get a spray explosion in the water then the torpedo went dud on collision, or it missed completely. Maybe passing under the ship without magnetic detection because a wave lifted it up.
Torpedo damage is random every time. So you may just have been unlucky that it wasn't enough to cross a certain damage visual threshold.
Depending on if/which mods you use, the most damage you do to a ship comes from flooding. That is something only time can show in how the waterline changes.
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