MidAir movement doesnt work, when i just jump without any starting ground speed

So, i have completed a tutorial series about a 2D platformer controller.

I edited it a bit, because some multiplier caused me a lot of bug, and chunky movement.
The movement is almost perfect for my game’s style, but there are a thing.

When i try to move in the mid air, the character only move when it has a starting speed in the ground.
so when i just jump, and then try to move i cant, but if i run and jump, i can move to the other direction.

here it is what i did:

{

    private float movementInputDirection;
    private float fJumpPressedRemember = 0;
    private float fJumpPressedRememberTime = 0.2f;
    private float fGroundedRemember = 0;
    private float fGroundedRememberTime = 0.2f;

    private int amountOfJumpsLeft;
    private int facingDirection = 1;

    private bool isFacingRight = true;
    private bool isWalking;
    private bool isGrounded;
    private bool isTouchingWall;
    private bool isWallSliding;
    private bool canJump;

    private Rigidbody2D rb;
    private Animator anim;

    public int amountOfJumps = 1;

    public float movementSpeed = 10.0f;
    public float jumpForce = 16.0f;
    public float groundCheckRadius;
    public float wallCheckDistance;
    public float wallSlideSpeed;
    public float moveForceInAir;
    //public float airDragMultiplier = 0.95f;
    //public float variableJumpHeightMultiplier = 0f;
    public float wallHopForce;
    public float wallJumpForce;

    public Vector2 wallHopDirection;
    public Vector2 wallJumpDirection;

    public Transform groundCheck;
    public Transform wallCheck;

    public LayerMask whatIsGround;

    // Start is called before the first frame update
    void Start()
    {
        rb = GetComponent<Rigidbody2D>();
        anim = GetComponent<Animator>();
        amountOfJumpsLeft = amountOfJumps;
        wallHopDirection.Normalize();
        wallJumpDirection.Normalize();
    }

    // Update is called once per frame
    void Update()
    {
        CheckInput();
        CheckMovementDirection();
        UpdateAnimations();
        CheckIfCanJump();
        CheckIfWallSliding();
    }


    private void FixedUpdate()
    {
        ApplyMovement();
        CheckSurroundings();
    }

    private void CheckIfWallSliding()
    {
        if (isTouchingWall && !isGrounded && rb.velocity.y < 0)
        {
            isWallSliding = true;
        }
        else
        {
            isWallSliding = false;
        }
    }

    private void CheckSurroundings()
    {
        isGrounded = Physics2D.OverlapCircle(groundCheck.position, groundCheckRadius, whatIsGround);

        isTouchingWall = Physics2D.Raycast(wallCheck.position, transform.right, wallCheckDistance, whatIsGround);
    }

    private void UpdateAnimations()
    {
        anim.SetBool("isWalking", isWalking);
        anim.SetBool("isGrounded", isGrounded);
        anim.SetFloat("yVelocity", rb.velocity.y);
        anim.SetBool("isWallSliding", isWallSliding);
    }


    private void CheckIfCanJump()
    {
        if ((isGrounded && rb.velocity.y <= 0) || isWallSliding)
        {
            amountOfJumpsLeft = amountOfJumps;
        }
        
        if (amountOfJumpsLeft <= 0)
        {
            canJump = false;
        }
        else
        {
            canJump = true;
        }
    }

    private void CheckMovementDirection() //Karakter direction-t lekérdezi
    {
        if(isFacingRight && movementInputDirection < 0)
        {
            Flip();
        }
        else if(!isFacingRight && movementInputDirection > 0)
        {
            Flip();
        }

        if(rb.velocity.x != 0)
        {
            isWalking = true;
        }
        else
        {
            isWalking = false;
        }
    }


    private void CheckInput() //Inputok
    {
        movementInputDirection = Input.GetAxisRaw("Horizontal");

        fJumpPressedRemember -= Time.deltaTime;
        if (Input.GetButtonDown("Jump"))
        {
            fJumpPressedRemember = fJumpPressedRememberTime;
            Jump();
        }

        fGroundedRemember -= Time.deltaTime;
        if (isGrounded)
        {
            fGroundedRemember = fGroundedRememberTime;
        }
        //if (Input.GetButtonUp("Jump"))
        //{
            //rb.velocity = new Vector2(rb.velocity.x, rb.velocity.y * variableJumpHeightMultiplier);
        //}
    }


    private void Jump()
    {
        if (canJump && !isWallSliding && (fJumpPressedRemember > 0) && (fGroundedRemember > 0))
        {
            fGroundedRemember = 0;
            fJumpPressedRemember = 0;
            rb.velocity = new Vector2(rb.velocity.x, jumpForce);
            amountOfJumpsLeft--;
        }
        else if (isWallSliding && movementInputDirection == 0 && canJump) //wall hop
        {
            isWallSliding = false;
            amountOfJumpsLeft--;
            Vector2 forceToAdd = new Vector2(wallHopForce * wallHopDirection.x * -facingDirection, wallHopForce * wallHopDirection.y);
            rb.AddForce(forceToAdd, ForceMode2D.Impulse);
        }
        else if ((isWallSliding || isTouchingWall)&& movementInputDirection != 0 && canJump)
        {
            isWallSliding = false;
            amountOfJumpsLeft--;
            Vector2 forceToAdd = new Vector2(wallJumpForce * wallJumpDirection.x * movementInputDirection, wallJumpForce * wallJumpDirection.y);
            rb.AddForce(forceToAdd, ForceMode2D.Impulse);
        }
    }

    private void ApplyMovement()
    {
        if (isGrounded)
        {
            rb.velocity = new Vector2(movementSpeed * movementInputDirection, rb.velocity.y);
        }
        else if (!isGrounded && !isWallSliding && movementInputDirection != 0)
        {
            Vector2 forceToAdd = new Vector2(moveForceInAir * movementInputDirection, 0);
            rb.AddForce(forceToAdd);

            if (Mathf.Abs(rb.velocity.x) > movementSpeed)
            {
                rb.velocity = new Vector2(movementSpeed * movementInputDirection, rb.velocity.y);
            }
        }
        else if (!isGrounded && !isWallSliding && movementInputDirection == 0)
        {
            //rb.velocity = new Vector2(rb.velocity.x * airDragMultiplier, rb.velocity.y);
            rb.velocity = new Vector2(rb.velocity.x * moveForceInAir, rb.velocity.y);
        }

        if (isWallSliding)
        {
            if(rb.velocity.y < -wallSlideSpeed)
            {
                rb.velocity = new Vector2(rb.velocity.x, -wallSlideSpeed);
            }
        }
    }


    private void Flip()
    {
        if (!isWallSliding)
        {
        facingDirection *= -1;
        isFacingRight = !isFacingRight;
        transform.Rotate(0.0f, 180.0f, 0.0f);
        }
        
    }


    private void OnDrawGizmos()
    {
        Gizmos.DrawWireSphere(groundCheck.position, groundCheckRadius);

        Gizmos.DrawLine(wallCheck.position , new Vector3(wallCheck.position.x + wallCheckDistance, wallCheck.position.y, wallCheck.position.z));
    }
}

private void ApplyMovement()
{
if (isGrounded)
{
rb.velocity = new Vector2(movementSpeed * movementInputDirection, rb.velocity.y);
}
else if (!isGrounded && !isWallSliding && movementInputDirection != 0)
{
Vector2 forceToAdd = new Vector2(moveForceInAir * movementInputDirection, 0);
rb.AddForce(forceToAdd);

             if (Mathf.Abs(rb.velocity.x) > movementSpeed)
             {
                 rb.velocity = new Vector2(movementSpeed * movementInputDirection, rb.velocity.y);
             }
         }
         else if (!isGrounded && !isWallSliding && movementInputDirection == 0)
         {
             //rb.velocity = new Vector2(rb.velocity.x * airDragMultiplier, rb.velocity.y);
             rb.velocity = new Vector2(rb.velocity.x * moveForceInAir, rb.velocity.y);
         }
 
         if (isWallSliding)
         {
             if(rb.velocity.y < -wallSlideSpeed)
             {
                 rb.velocity = new Vector2(rb.velocity.x, -wallSlideSpeed);
             }
         }
     }

If the problem is that you can’t move while jumping, this is probably where the problem is in. You are not grounded while jumping, not wallsliding and applying input, so the first else if statement is where the problem should be. movementInputDirection is used in other parts so can’t be the problem. So my guess is that moveForceInAir is not assigned and forceToAdd will just be Vector2.zero. It’s also only used in this part of the script.