Trouble with Stamina controller

This time I was following the steps of a tutorial (https://youtu.be/teaMf4hC16k) to make an Health / Mana / Stamina bar in my game, and I have two problems. The first problem is that the "&&" logic gate doesn't work, and the Stamina falls only pressing "LShift. The second is that when Stamina goes 0, the character can continue sprinting.I revised the whole code for 2 hours and i haven't found the issue. Here's the whole code:

var currentHealth : float = 39; var maxHealth : int = 39;

var currentMana : float = 50.0; var maxMana : int = 50;

var currentStamina : float = 50.0; var maxStamina : float = 50;

var barLength = 0.0;

private var chMotor : CharacterMotor;

function Start () { barLength = Screen.width / 8; chMotor = GetComponent(CharacterMotor); }

function Update () { AdjustCurrentHealth (0); AdjustCurrentMana (0);

 /*MANA CONTROL SECTION*/
 
 //Normal Mana Regeneration
 if (currentMana >= 0) {
     currentMana += Time.deltaTime * 2;
 }
 
 //Don't let Mana go above 100
 if (currentMana >= maxMana) {
     currentMana = maxMana;
 }
 
 //Don't let Mana go below 0
 if (currentMana <= 0) {
     currentMana = 0;
 }
 
 //When does Mana decrease?
 if (Input.GetKeyDown("q")) {
     AdjustCurrentMana (-20);
 }
 
 if (Input.GetKeyDown("e")) {
     AdjustCurrentMana (-25);
 }
 
 /*STAMINA CONTROL SECTION*/
 
 //Set and find Character Controller
 var controller : CharacterController = GetComponent (CharacterController);
 
 //Reduce Stamina bar when pressing LShift and moving
 if (controller.velocity.magnitude > 0 && Input.GetKey(KeyCode.LeftShift)) {
     currentStamina -= Time.deltaTime * 10;
     chMotor.movement.maxForwardSpeed = 10;
     chMotor.movement.maxSidewaysSpeed = 10;
 }
 
 //Normal speed when not pressing anything
 else {
     chMotor.movement.maxForwardSpeed = 6;
     chMotor.movement.maxSidewaysSpeed = 6;
 }
 
 //Stamina regeneration
 if (controller.velocity.magnitude == 0 && (currentStamina >= 0)) {
     currentStamina += Time.deltaTime * 10;
 }
 
 //Don't let sprint when Stamina is less than 0
 if (controller.velocity.magnitude > 0 && Input.GetKey(KeyCode.LeftShift) && currentStamina <= 0) {
     chMotor.movement.maxForwardSpeed = 6;
     chMotor.movement.maxSidewaysSpeed = 6;
 }
 
 //Don't let Stamina go above 100
 if (currentStamina >= maxStamina) {
     currentStamina = maxStamina;
 }
     
 //Don't let Stamina go below 0
 if (currentStamina <= 0) {
     currentStamina = 0;
 }

}

function OnGUI () { //Icons for GUI GUI.Box(new Rect(5, 30, 40, 20), "HP"); GUI.Box(new Rect(5, 50, 40, 20), "Mana"); GUI.Box(new Rect(5, 70, 40, 20), "Stam");

 //Health / Mana / Stamina main bars
 GUI.Box(new Rect(45, 30, barLength, 20), currentHealth.ToString("0") + "/" + maxHealth);
 GUI.Box(new Rect(45, 50, barLength, 20), currentMana.ToString("0") + "/" + maxMana);
 GUI.Box(new Rect(45, 70, barLength, 20), currentStamina.ToString("0") + "/" + maxStamina);

}

function AdjustCurrentHealth (adj) { currentHealth += adj;

 if(currentHealth >= maxHealth) 
 {
     currentHealth = maxHealth;
 }
 
 if (currentHealth <= 0) 
 {
     currentHealth = 0;
 }

}

function AdjustCurrentMana (adj) { currentMana += adj; }

And this is the Stamina Section:

/STAMINA CONTROL SECTION/

 //Set and find Character Controller
 var controller : CharacterController = GetComponent (CharacterController);
 
 //Reduce Stamina bar when pressing LShift and moving
 if (controller.velocity.magnitude > 0 && Input.GetKey(KeyCode.LeftShift)) {
     currentStamina -= Time.deltaTime * 10;
     chMotor.movement.maxForwardSpeed = 10;
     chMotor.movement.maxSidewaysSpeed = 10;
 }
 
 //Normal speed when not pressing anything
 else {
     chMotor.movement.maxForwardSpeed = 6;
     chMotor.movement.maxSidewaysSpeed = 6;
 }
 
 //Stamina regeneration
 if (controller.velocity.magnitude == 0 && (currentStamina >= 0)) {
     currentStamina += Time.deltaTime * 10;
 }
 
 //Don't let sprint when Stamina is less than 0
 if (controller.velocity.magnitude > 0 && Input.GetKey(KeyCode.LeftShift) && currentStamina <= 0) {
     chMotor.movement.maxForwardSpeed = 6;
     chMotor.movement.maxSidewaysSpeed = 6;
 }
 
 //Don't let Stamina go above 100
 if (currentStamina >= maxStamina) {
     currentStamina = maxStamina;
 }
     
 //Don't let Stamina go below 0
 if (currentStamina <= 0) {
     currentStamina = 0;
 }

}

Any ideas to solve this problem? Thanks in advance!