Odd Capsule Collider Behavior

I'm trying to use the OnMouseDown() function on a capsule that has a capsule collider. For some reason, it only detects the mouse down event when I click on the round ends of the capsule, and not when I click on the cylindrical portion.

The script also handles OnMouseDown() on two other left and right arrow objects. The arrows behave as intended, but not the capsule.

 void OnMouseDown() {

     if (objectName == "arrowLeft") {
         Debug.Log ("left arrow clicked");
         transform.Translate (new Vector3(-0.2f, 0, 0));

     }
     else if (objectName == "arrowRight") {
         Debug.Log ("right arrow clicked");
         transform.Translate (new Vector3(0.2f, 0, 0));
     }
     else if (objectName == "Capsule") {
         Debug.Log ("Capsule clicked");
         float r = Random.Range (0f, 1f);
         float g = Random.Range (0f, 1f);
         float b = Random.Range (0f, 1f);
         Color randomColor = new Color (r, g, b, 1f);
         renderer.material.color = randomColor;
     }
 }