CharacerController not allowing mid-air movement

I am attempting to implement a 3rd person platformer. A key aspect of platforming is the ability to control your character while in the air. I took the code from the Unity documentation on CharacterController.Move and I noticed that if I took the movement part of the code out of the if(player.isGrounded) (so that the player could still move even if he was in the air), I couldn't jump. I tried a lot of things (changing values, lerping positions, and lots more) but I couldn't get the jumping working. Here's the code:

 using UnityEngine;
 using System.Collections;

 public enum State
 {
     standing = 0,
     walking = 1,
     running = 2,
     sprinting = 3,
     jumping = 4
 }

 [RequireComponent (typeof(CharacterController))]
 public class MovementController : MonoBehaviour
 {
     public float walkSpeed = 3.0f;
     public float runSpeed = 4.0f;
     public float sprintSpeed = 6.0f
     public float strafeSpeed = 0.80f;

     public float runThreshold = 0.5f;
     public float maxSprintSeconds = 15.0f;
     public float sprintCooldown = 1.0f;
     public float sprintCooldownStart = 5.0f;

     public float jumpForce = 8.0f;
     public float airControl = 3.0f;
     public float gravity = 20.0f;

     private Vector3 moveDirection = Vector3.zero;
     private CharacterController player;
     private float horizontal;
             private float vertical;
             private State currentState = State.standing;

     public void Start ()
     {
         player = GetComponent<CharacterController> ();

         if (!player) {
             Debug.Log ("There is no CharacterController component attached to the \"" + gameObject.name +
                 "\", and the MovementController.cs script depends on it!");
         }
     }

     public void Update ()
     {
         if (!player)
             return;

         horizontal = Input.GetAxis ("Horizontal");
         vertical = Input.GetAxis ("Vertical");

         if (player.isGrounded) {
             if (horizontal == 0 && vertical == 0)
                 currentState = State.standing;
             else if (Mathf.Abs (horizontal) < runThreshold && Mathf.Abs (vertical) < runThreshold)
                 currentState = State.walking;
             else if (Mathf.Abs (horizontal) > runThreshold || Mathf.Abs (vertical) > runThreshold)
                 currentState = State.running;

             if (currentState == State.walking) {
                 moveDirection.Set (horizontal * strafeSpeed * walkSpeed, 0, vertical * walkSpeed);
             }
             if (currentState == State.running) {
                 moveDirection.Set (horizontal * strafeSpeed * runSpeed, 0, vertical * runSpeed);
             }

             if (Input.GetButton ("Jump"))
                 moveDirection.y = jumpForce;
         } else {
             currentState = State.jumping;

             moveDirection.x = horizontal * strafeSpeed * airControl;
             moveDirection.y = vertical * airControl;
         }

         moveDirection = transform.TransformDirection (moveDirection);
         moveDirection.y -= gravity;
         moveDirection *= Time.deltaTime;

         player.Move (moveDirection);
     }

 }

Thanks in advance, for helping me. :)