The convoy has to be in sight ... visual range... radar contacts dont count.
Typically you have to sight more than one ship.
You have to be surfaced.
The icon for report contact shall not be 'ripped' in half otherwise you havent met the other pre-requirements.
Note that being in visual range, sighting more than one ship, while surfaced, are not exactly a safe way to be conducting oneself in a time of war
