Problem with Jump Animation

Hey Guys.

I'm coding a 2DUnity Game. I'm really new to coding, so i need your help.

If i Jump the Character Jumps but don't show the Jump animation. It only shows the Jump animation for 1 Frame. I think it's something with the groundchecker, but i don't know how to fix it.

The Code is: using System.Collections; using System.Collections.Generic; using UnityEngine;

public class PlayerMovement : MonoBehaviour {

     public CharacterController2D controller;
     public Animator animator;

     public float runSpeed = 40f;

     float horizontalMove = 0f;
     bool jump = false;
     bool crouch = false;

      // Update is called once per frame
      void Update () {

              horizontalMove =  Input.GetAxisRaw("Horizontal") * runSpeed;

              animator.SetFloat("Speed", Mathf.Abs(horizontalMove));

              if (Input.GetButtonDown("Jump"))
              {
                jump = true;
                animator.SetBool("IsJumping", true);
              }

              if (Input.GetButtonDown("Crouch"))
              {
                    crouch = true;
              } else if (Input.GetButtonUp("Crouch"))
              {
                crouch = false;
              }
       }

       public void OnLanding ()
       {
         animator.SetBool("IsJumping", false);
       }

       public void OnCrouching (bool isCrouching){

           animator.SetBool("IsCrouching", isCrouching);
       }

       void FixedUpdate ()
       {
                // Move our character
                controller.Move(horizontalMove * Time.fixedDeltaTime, crouch, jump);
                jump = false;
       }
     }