Problem With Lock the Levels

hi...i made code for my level select screen for when i completed level 1 the lock image on the level 2 will disappear...im starting to playing level 2 and compelete that either...it will unlock the level 3 for me but lock the level 2 again after i finish that...and it happens for the next levels aswell...where is the problem?

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class LevelMenu : MonoBehaviour {
 
     public Button btn1, btn2, btn3;
 
     public GameObject Lock1, Lock2, Lock3;
 
     // Use this for initialization
     void Start () {
 
         PlayerPrefs.SetString ("lo1", "On");
 
         if (PlayerPrefs.GetString ("lo1") == "On")
             Lock1.SetActive (false);
 
         if (PlayerPrefs.GetString ("lo2") == "On")
             Lock2.SetActive (false);
 
         if (PlayerPrefs.GetString ("lo3") == "On")
             Lock3.SetActive (false);
 
         btn1.onClick.AddListener (()=>{
 
             if(PlayerPrefs.GetString ("lo1") == "On")
                 Application.LoadLevel ("lo1");
             });
 
             btn2.onClick.AddListener (()=>{
 
                 if(PlayerPrefs.GetString ("lo2") == "On")
                 Application.LoadLevel ("lo2");
                 });
 
                 btn3.onClick.AddListener (()=>{
 
                     if(PlayerPrefs.GetString ("lo3") == "On")
                 Application.LoadLevel ("lo3");
             });
         
     }
     
     // Update is called once per frame
     void Update () {
         
     }
 }
 

i also added this code to my HUD script where it tells you win the game:

 public void OnGameWin(int score)
     {
         gameOver.ShowWin (score, starIdx);
         if (starIdx > PlayerPrefs.GetInt (UnityEngine.SceneManagement.SceneManager.GetActiveScene ().name, 0)) {
             PlayerPrefs.SetInt (UnityEngine.SceneManagement.SceneManager.GetActiveScene ().name, starIdx);
             PlayerPrefs.SetString (NextLevel, "On");