Player not double jumping.

Hello! I made this script for double jumping.But my player does not double jump,but it can jump in the air once if it falls off the ground. here's the script:-

using UnityEngine; using System.Collections;

public class PlayerControls : MonoBehaviour {

 Rigidbody rb;

 public bool IsGrounded ;
 public bool can2j ;
 public float jumpHeight;
 public float jumpSpeed;
 Animator anim;
 
 void Start(){
     anim = GetComponent<Animator> ();
     rb = GetComponent<Rigidbody> ();

 }
 void FixedUpdate ()
 {
     if (IsGrounded) {
         can2j = false;
     } 
     if (Input.GetKeyDown (KeyCode.LeftControl)) {

         anim.SetBool ("attack", true);
     } else  {
         anim.SetBool ("attack", false);
     }
     if (Input.GetKeyDown (KeyCode.Space) && IsGrounded){
         
             rb.velocity = new Vector3 (0, jumpHeight, 0);
             IsGrounded = false;
             can2j = false;
         } 
     if(Input.GetButtonDown("Jump")) {
         if(!IsGrounded&& !can2j){
             rb.velocity = new Vector3 (0, jumpSpeed, 0);
             can2j = true;
         }
     }

         float moveH = Input.GetAxis ("Horizontal");
         Vector3 movement = new Vector3 (moveH * 20, rb.velocity.y, 0f);
         rb.velocity = movement; 
         if (moveH < 0) {

             rb.rotation = Quaternion.Euler (0, 180, 0);
         }  
         if (moveH > 0 && transform.eulerAngles.y == 180) {
         
             rb.rotation = Quaternion.Euler (0, 0, 0);
         }
     }
 void OnCollisionEnter(Collision other){

     if (other.gameObject.tag == "Ground") {
         IsGrounded = true;
     }
 }
     void OnCollisionExit (Collision other)
 {

     if (other.gameObject.tag == "Ground") {
         IsGrounded = false;

     }
 }void OnCollisionStay(Collision other){
     if (other.gameObject.tag == "Ground") {
         IsGrounded = true;
     }

 }

 }