Dark Waters, submarine school torpedo lessons...

As a Dark Waters novice I have now started with torpedo attack in the submarine school. The first ship I found I successfully torpedoed.

A little later I received the radio message that I had to sail to a second point where I, within a radius of 35 km., should find a second ship to torpedo too.

I've done that several times now, but that second ship is nowhere to be found!

Have you also experienced this? Is there a solution? Thank you!