Raycast onto Mesh Collider unreliable.

I'm trying to make an RTS game for which I created the map myself as a mesh in Blender. My map is made up of separate convex parts, each equipped with a convex mesh collider. The plane under all these parts has a box collider, which reliably receives the raycasts. I have the following code for finding the clicked position on the map, which works only sometimes on the mesh colliders.

The question: How can I make the raycasts hit the mesh colliders reliably? Thank you for any help in advance.

 Ray ray = mainCamera.ScreenPointToRay (Input.mousePosition);
 RaycastHit[] hits;
 hits = Physics.RaycastAll (ray.origin, ray.direction, clickDistance);
 foreach (RaycastHit hit in hits) {
     if (hit.collider.gameObject.tag == "Map") {
         return hit.point;
     }
 }
 return Vector3.zero;