Why wont the sprinting on this code work?

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent (typeof(CharacterController))]
 
 public class FirstPersonController : MonoBehaviour {
     
     public float mouseSensitivity = 5.0f;
     public float jumpSpeed = 20.0f;
     public float upDownRange = 60.0f;
     public float walkSpeed = 5.0f;
     public float sprintSpeed = 10.0f;
     
     float verticalVelocity = 0;
     float verticalRotation = 0;
     float actualSpeed;
 
     
     CharacterController characterController;
     
     
     // Use this for initialization
     void Start () {
         
         Screen.lockCursor = true; // Hides the cursor on playing
         characterController = GetComponent<CharacterController> (); //Retrieves CC 
         
     }
     
     // Update is called once per frame
     void Update () {
         
     
         // ** Movement of Player **
 
         actualSpeed = walkSpeed;
         float forwardSpeed = Input.GetAxis ("Vertical") * actualSpeed; //Movement speed of player - forward movement
         float sideSpeed = Input.GetAxis ("Horizontal") * actualSpeed / 2; //Movement speed of player - side movement
         
                 
         verticalVelocity += Physics.gravity.y * Time.deltaTime; //Applies acceleration p/s of Gravity when falling
         
         // Restricts player from jumping twice in mid-air                                                                                                                                                               ir until player hits ground
         
         if ( characterController.isGrounded && Input.GetButtonDown ("Jump")) {
             verticalVelocity = jumpSpeed;        
         }
 
         //Allows sprinting
 
         if (characterController.isGrounded && Input.GetKeyDown(KeyCode.LeftShift)) {
             actualSpeed = sprintSpeed;
         }
         if (characterController.isGrounded && Input.GetKeyUp(KeyCode.LeftShift)) {
             actualSpeed = walkSpeed;
         }
 
         Vector3 speed = new Vector3 ( sideSpeed, verticalVelocity, forwardSpeed ); //Vector for movement of the player
         
         speed = transform.rotation * speed; // ?
         
         
         characterController.Move ( speed * Time.deltaTime ); //Applies speed of player p/frame
         
     }
     
 }

Why wont this code work? In the //Allows sprinting, i have the if statement to check whether the person has pressed the left shift, and if they have, change the actual speed of the player (5, stated at the beginning of //Movement of player) to the sprint speed (10). I don't know what's wrong D: Help plz.