Sphere objects pass through terrain trees

Hi all,

i have a problem with collisions. i use FPS and attached a launcher to its camera, the launcher fire spheres as if there missile. the spheres prefab has its Sphere Collider and it collides with other objects, but they pass through the terrain trees. Also, i made the "Create Tree Colliders" in the terrain options checked, yet the spheres still don collide

so how to make my objects collide with the trees ?

this is the line i use to instantiate the spheres:

 if (Input.GetButtonDown("Fire1")) {
     // instantiate a new object of the type of the missile
     var instantiatedProjectTile : Rigidbody = Instantiate(projectTile, transform.position, transform.rotation);
     // adjust the velocity of the missle in the Z direction
     instantiatedProjectTile.velocity = transform.TransformDirection(Vector3(0, 0, speed));
     // use Physics to stop collision between the missile and the player
     Physics.IgnoreCollision(instantiatedProjectTile.collider, transform.root.collider);
 }

this script is attached to the FPS camera

any help

Thanks,

Samer Samy