Stamina doesnt always activate

Im trying to make a stamin system in my game but it doesnt really work. I am able to run by hitting shift which is the sprint button, but the stamina doesn't deplete until a few secoinds later. It sometimes doesn't activate at all. How can I fix this?

{

 // groundCheck
 public Transform groundCheck;
 public float groundDistance = 1;
 public LayerMask groundMask;
 private bool isGrounded;
 //move
 public CharacterController controller;
 public float speed = 8;
 Vector3 velocity;
 public float gravity = -9.81f;
 public float jumpHeight;
 //health and stamina
 public Slider HealthBar;
 public Slider StaminaBar;
 public int currentStamina;
 public int maxStamina = 100;
 public int maxHealth = 100;
 public int currentHealth;
 public Stamina stamina;
 public bool ifRun = false;
 public bool ifWalk = true;
  void Start()
 {
     currentStamina = maxStamina;
     stamina.SetMaxStamina(maxStamina);
     currentHealth = maxHealth;

     
 }


 void Update()
 {
     //gravity
     isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);
     if (isGrounded && velocity.y < 0)
     {
         velocity.y = -2f;

     }
     //move
     float X = Input.GetAxis("Horizontal");
     float Z = Input.GetAxis("Vertical");
     Vector3 move = transform.right * X + transform.forward * Z;

     controller.Move(move* speed * Time.deltaTime);

     velocity.y += gravity * Time.deltaTime;
     
     controller.Move(velocity * Time.deltaTime);
     //sprint
     if (X > 0 || X < 0 || Z < 1 || Z > -1)
     {
         ifWalk = true;
     }
     if (ifWalk == true && Input.GetKey("left shift"))
     {
         UseStamina(2);
         speed = 10;
         ifRun = true;

     }
     else if (Input.GetKeyUp("left shift"))
     {
         speed = 5;
         ifRun = false;

     }
     if (ifRun == false)
     {
         UseStamina(-5);
     }



     //jump
     if (Input.GetButtonDown("Jump") && isGrounded == true)
     {
         velocity.y = Mathf.Sqrt(jumpHeight * -2 * gravity);
     }

     

 }
  public void UseStamina(int amount)
 {
     
     if (currentStamina - amount >= 0)
     {
         currentStamina -= amount;
         stamina.setStamina(currentStamina);
     }
 }

}