Game test-build sometimes not recognising key inputs?

Hey guys, I'm working on a simple third person game. In the game you can talk to a NPC. This is how it works:

The players script does a Physics.OverlapSphere() to check if it's near the NPC.

If he is near, then it waits for Input.GetKeyDown(keycode.Return) to initiate a very simple dialogue system.

This brings up a simple GUI which displays the message.

When I play the game in the editor window, everything works perfectly. However, after test-building the game, this only works about 50% of the time. It appears that the key press does not register.

My player only has one script attached, so it is quite long. Would rewriting it into separate classes make it more efficient? I have been using Unity for over a year now. I am alright at scripting, but I learned through trial and error so I may be missing something important.

Here is a copy of my players script:

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent(typeof(CharacterController))]
 
 public class Player : MonoBehaviour {
     //Player control keycodes
     public KeyCode forward = KeyCode.W;
     public KeyCode left = KeyCode.A;
     public KeyCode right = KeyCode.D;
     public KeyCode jump = KeyCode.Space;
     //Action keycodes
     public KeyCode action = KeyCode.Return;
     //GUI
     public GUISkin display;
     
     //CharacterController component
     private CharacterController controller;
     //Player movement variables
     private bool canMove = true;
     private Vector3 moveDir = Vector3.zero;
     private float moveSpeed = 6.0f;
     private float turnSpeed = 3.0f;
     private float jumpForce = 8.0f;
     private float gravForce = -20.0f;
     //Player statistics
     private int maxHealth = 3;
     private int curHealth;
     private int gems;
     private float showDelay = 2.5f;
     private float showStats = 0.0f;
     //Actions
     private float sensorRange = 2.0f;
     private string pickupTag = "Pickup";
     private GameObject pickup;
     private string npcTag = "NPC";
     private GameObject npc;
     private bool nearNpc = false;
     private bool isTalking = false;
     private string curMessage;
     private int messageNumber = 0;
     //Screen resolution and GUI
     private float originalWidth = 1024.0f;
     private float originalHeight = 768.0f;
     private float displayX;
     private float displayY;
     private float speechBoxX;
     private float speechBoxY;
     
     void Start() {
         controller = gameObject.GetComponent<CharacterController>();
         SetGUI(originalWidth, originalHeight);
     }
     
     void Update() {
         if(controller.isGrounded) {
             if(canMove) {
                 Move();
             }
             if(nearNpc) {
                 if(Input.GetKeyDown(action)) {
                     Talk();    
                 }
             }
         }
         senseAction();
         moveDir.y += gravForce * Time.deltaTime;
         controller.Move(moveDir * Time.deltaTime);
     }
     
     void Move() {
         if(Input.GetKey(forward)) {
             moveDir = new Vector3(0.0f, 0.0f, moveSpeed);
             moveDir = transform.TransformDirection(moveDir);
         }
         else {
             moveDir = new Vector3(0.0f, 0.0f, 0.0f);
             moveDir = transform.TransformDirection(moveDir);
         }
         if(Input.GetKey(right)) {
             transform.Rotate(0.0f, turnSpeed * 90.0f * Time.deltaTime, 0.0f);
         }
         if(Input.GetKey(left)) {
             transform.Rotate(0.0f, -turnSpeed * 90.0f * Time.deltaTime, 0.0f);
         }
         if(Input.GetKeyDown(jump)) {
             moveDir.y = jumpForce;
         }
     }
     
     void senseAction() {
         Vector3 origin = transform.position + controller.center;
         Collider[] hitColliders = Physics.OverlapSphere(origin, sensorRange);
         for(int i = 0; i < hitColliders.Length; i++) {
             if(hitColliders[i].tag == pickupTag) {
                 pickup = hitColliders[i].gameObject;
                 pickup.SendMessage("AttractGem", true, SendMessageOptions.RequireReceiver);
             }
             if(hitColliders[i].tag == npcTag) {
                 npc = hitColliders[i].gameObject;
                 nearNpc = true;
             }
             else {
                 nearNpc = false;    
             }
         }    
     }
     
     void Talk() {
         NPC dialogue = npc.GetComponent<NPC>();
         if(messageNumber < dialogue.speech.Length) {
             isTalking = true;
             canMove = false;
             moveDir = new Vector3(0.0f, 0.0f, 0.0f);
             moveDir = transform.TransformDirection(moveDir);
             showStats = showDelay;
             curMessage = dialogue.speech[messageNumber];
             messageNumber ++;
         }
         else {
             isTalking = false;
             canMove = true;
             messageNumber = 0;
         }
     }
     
     public void AdjHealth(int adj) {
         curHealth += adj;
         if(curHealth <= 0) {
             curHealth = 0;
             StartCoroutine(Die());
         }
         if(curHealth > maxHealth) {
             curHealth = maxHealth;    
         }
         showStats = showDelay;
     }
     
     public void AdjGems(int adj) {
         gems += adj;
         if(gems < 0) {
             gems = 0;    
         }
         showStats = showDelay;
     }
     
     IEnumerator Die() {
         canMove = false;
         yield return new WaitForSeconds(1.0f);
         Debug.Log("You Are Dead");
     }
     
     void SetGUI(float width, float height) {
         displayX = width * 0.20f;
         displayY = height * 0.05f;
         speechBoxX = width * 0.5f;
         speechBoxY = height * 0.15f;
     }
     
     void OnGUI() {
         Vector2 ratio = new Vector2(Screen.width/originalWidth , Screen.height/originalHeight);
           Matrix4x4 guiMatrix = Matrix4x4.identity;
           guiMatrix.SetTRS(new Vector3(1.0f, 1.0f, 1.0f), Quaternion.identity, new Vector3(ratio.x, ratio.y, 1.0f));
           GUI.matrix = guiMatrix;
         
         GUI.skin = display;
         
         if(showStats > 0.0f) {
             GUI.Box(new Rect(25.0f, 25.0f, displayX, displayY), "Health: " + curHealth);
             GUI.Box(new Rect(originalWidth - displayX - 25.0f, 25.0f, displayX, displayY), "Gems: " + gems);
             if(!isTalking) {
                 showStats -= Time.deltaTime;
             }
         }
         
         if(isTalking) {
             GUI.Box(new Rect(originalWidth * 0.5f - speechBoxX * 0.5f, originalHeight - speechBoxY - 25.0f, speechBoxX, speechBoxY), curMessage + "\n\nPress Action To Continue");    
         }
         
         GUI.matrix = Matrix4x4.identity;
     }
 }

Thanks for your time.