Terrain collider not working with gameObject:

Unity Engine
1

I have a small android game going. The terrain is 15x40 and I have a collider on it and istrigger is unchecked. The gameobject has a rigidbody and colliders on it.

When the object is thrown it goes through the terrian in some spots and collides with the terrain in other spots.

This is the code to throw the object.

Transform thrownItem = Instantiate(_itemPrefab, transform.position, transform.rotation) as Transform;
            thrownItem.name = _itemPrefab.name;
            
            thrownItem.rigidbody.AddRelativeForce(thrownItem.forward * _throwForce, ForceMode.Impulse);
2

Any ideas how this is happening???