Object not always hitting ground with gravity

Some objects collide with the car I am using and therefore sit on the bonnet, when I drive away they stay at bonnet height.
How do I make it drop after I drive away?
Using a rigidbody and gravity I thought would be enough but apparently its not..

Any Ideas

 

That should be more than enough. Have you paused the game and had a look at the object that remains in the air? Is its collider in the correct position? If it is on a child object with a Rigidbody, it might have become "dislodged".