The cause of the problem was not 'immediately' clear. [=is was not known right away]
They live in the house 'immediately' [='directly'] behind this one.
We need to leave 'immediately'. [='at once', 'right away']
(..) Starting its month-long deceleration to intercept. But instead, what I'm proposing is... We start accelerating immediately to preserve velocity and gain even more. We don't intercept with earth at all... But we come close enough (..)2015 The Martian