My jumping in my game is inconsistent

My characters jumps are inconsistent, sometimes I have to press spacebar multiple times for it to jump while other times it only takes one try. I assumed it was because I was moving so I added more functionality into the if statement in the handleInput function to try and cope with it, but this does not fix it. Another problem I am having is after I jump sometimes after I land I get stuck in place, but if I jump again and land it unsticks itself, very strange :( . Please help. Here is my code:

using UnityEngine; using System.Collections;

public class Player : MonoBehaviour { private Rigidbody2D RigidBody; //A private varible for RigidBody2D to be stored in

 private Animator Animation;
 //Varible to store the animator from unity

 [SerializeField]
 private float movementSpeed;
 //Varible to store speed of the player

 private bool FacingRight;

 [SerializeField]
 private Transform[] GroundPoints;

 [SerializeField]
 private float GroundRadius;

 [SerializeField]
 private LayerMask WhatIsGround;

 private bool isGrounded;

 private bool jump;

 [SerializeField]
 private bool AirControl;

 [SerializeField]
 private float jumpForce;


 // Use this for initialization
 void Start() {

     FacingRight = true;

     RigidBody = GetComponent<Rigidbody2D>();
     //Getting RigidBody2D and storing it in the RigidBody varible

     Animation = GetComponent<Animator>();
     //Getting animator from unity and storing it in the animation varible

 }

 // Update is called once per frame
 void FixedUpdate()
 {
     isGrounded = IsGrounded();

     float Horizontal = Input.GetAxis("Horizontal");
     //Creates a reference for Unity called Horizontal

     Movement(Horizontal);
     //Calls the movement function

     Flip(Horizontal);

     HandleInput();

 }
 private void Movement(float Horizontal)
 {
     if (isGrounded || AirControl)
     {
         RigidBody.velocity = new Vector2(Horizontal * movementSpeed, RigidBody.velocity.y);
     }
     //Makes the player go either left or right depending what key is pressed leaving the y unchanged
     Animation.SetFloat("Speed", Mathf.Abs(Horizontal));
     //Sets the Speed parameter in unity to the horizontal, however the math function makes sure it always
     //positive so it doesn't return a negative speed
     if (isGrounded && jump)
     {
         isGrounded = false;
         RigidBody.AddForce(new Vector2(0, jumpForce));
         jump = true;
         jump = false;
     }
 }

 private void HandleInput()
 {
     if ((Input.GetKeyDown(KeyCode.Space) && Input.GetKeyDown(KeyCode.LeftArrow)) || (Input.GetKeyDown(KeyCode.Space) && Input.GetKeyDown(KeyCode.RightArrow)) || ((Input.GetKeyDown(KeyCode.Space) && Input.GetKeyDown(KeyCode.D)) || (Input.GetKeyDown(KeyCode.Space) && Input.GetKeyDown(KeyCode.A)) || (Input.GetKeyDown(KeyCode.Space))))
         {
             jump = true;
             Debug.Log("Space key was pressed.");
         }
 }


 private void Flip(float Horizontal) {
     if (Horizontal > 0 && !FacingRight || Horizontal < 0 && FacingRight) {
         FacingRight = !FacingRight;
         // If the Sprite is facing right and the FacingRight varible is not true or
         //it is facing left and the FacingRight varible is set to true it will make the
         //FacingRight varible the opposite of what it was before ie. False to true or true to false

         Vector3 TheScale = transform.localScale;
         //Gets the scale from unity and stores it in a varible so it can refrenced

         TheScale.x *= -1;
         //The scale is made the inverse of what it was before (1 to -1 or -1 to 1) flipping the Sprite

         transform.localScale = TheScale;
     }
 }

 private bool IsGrounded()
 {
     if (RigidBody.velocity.y <= 0)
     //if the player is falling or still
     {
         foreach (Transform point in GroundPoints)
         {
             Collider2D[] Colliders = Physics2D.OverlapCircleAll(point.position, GroundRadius, WhatIsGround);

             for (int i = 0; i < Colliders.Length; i++)
             {
                 if (Colliders[i].gameObject != gameObject)
                 {
                     return true;
                 }
             }

         }
     }
     return false;
 }

}