Jumping stops after certain collisions

Hello! I am a new unity user and I was following a tutorial on Unity but the jump script was not working well. I have a sphere character. I've tried many solutions but none have worked. Basically, when my ball moves from one cube to another, it won't jump at all anymore. This also happens if I hit a slope or a ceiling. JavaScript script by the way. Script: #pragma strict

 var rotationSpeed = 100;
 var jumpHeight = 8;
 
 private var isFalling = false;
 
 function Update () 
 {
     //moves ball left and right
     var rotation : float = Input.GetAxis ("Horizontal") * rotationSpeed;
     rotation *= Time.deltaTime;
     rigidbody.AddRelativeTorque (Vector3.back * rotation);
     
     if (Input.GetKey(KeyCode.UpArrow) && isFalling == false)
     {
         rigidbody.velocity.y = jumpHeight;
         isFalling = true;
     }
 }
 
 function OnTriggerEnter()
 {
 isFalling = false;
 }
 function OnTriggerExit()
 {
 isFalling = true;
 }

A picture of one of the areas my jump breaks: http://puu.sh/9IzmO/6f9175bf9c.jpg Thanks in advance