Problem with jump script

You should not be taking user input during FixedUpdate(). Take inputs in Update().

Input.GetButtonDown() only returns true during the frame the button was pressed, so if FixedUpdate() occurred on any frames other than when the button was pressed, it will be false. Which is why your jump "sometimes" works.
Update() occurs every frame so that's why you should take inputs in Update().

Here's a solid explanation: http://gamedev.stackexchange.com/questions/73713/update-vs-fixedupdate-in-unity

And here's the fixed code:

 public class platform : MonoBehaviour 
 {
     public Rigidbody target;
     public int force = 250;
     public float speed = 0.1f;
     public int jumpHeight = 6;
     public bool isFalling = false;
     private bool isJumping = false;
 
  
     // Use this for initialization
     void Start () 
     {
  
     }
  
     // Update is called once per frame
     void Update () 
     {
  
         if(Input.GetKeyDown(KeyCode.R))
         {
             Application.LoadLevel(0);
         }
 
         if(Input.GetButtonDown("Fire1") && isFalling == false)
             isJumping = true;
 
     }
  
     // Update is called once per frame
     void FixedUpdate () 
     {
         ballMovements ();   
     }
  
     void ballMovements()
     {
         rigidbody.AddRelativeTorque(Vector3.right * -1 * speed);
  
         if( isJumping )
         {
             isJumping = false;
             Debug.Log ("clicked");
             Vector3 temp = rigidbody.velocity;
             temp.y = jumpHeight;
             rigidbody.velocity = temp;
         }
         isFalling = true;
     }
  
     void OnCollisionStay()
     {
         isFalling = false;
     }
 }