Scenes not loading in the correct order

Hi, I'm currently creating a basic unity 3D game and I have come across a problem with the scenes not loading in the correct order.

I have a main menu , 3 levels, a 'You win' and finally a 'You lose' scene (making a total of 6 scenes in that build order.)

However after clicking new game, it enters level one, upon completing level 1 it repeats level 1, then upon completing level 1 again it will successfully load level 2, and then upon completing level 2 it jumps straight to the 'You win' scene. I am new to unity and coding in general and feel i am missing something relatively obvious but i have been looking for hours for a solution and found nothing. Any help would be appreciated!

Here is the GameManager part involved:

using UnityEngine; using UnityEngine.SceneManagement; using System.Collections; public class GameManager2 : MonoBehaviour{

 public int currentScore;
 public int highscore;
 public static int currentLevel = 0;
   
 void Update()

 {  
     startTime -= Time.deltaTime;
     currentTime = string.Format("{0:0.0}", startTime);
     if (startTime <= 0)
     {
         startTime = 0;
         SceneManager.LoadScene(5);
     }
 }

   
 public void CompleteLevel()
 {
     if (currentLevel < 4)
     {  
         //this loads levels in sequence
         SceneManager.LoadScene(currentLevel += 1);

     
     }
     else {

         //this is the win settings
         SceneManager.LoadScene(4);
         print("You win");
     }
 } 

Here is the MainMenu part involved:

 public void LoadScene(string name)

{

     SceneManager.LoadScene(name); 

}

 public void QuitGame() { 

     Application.Quit();
 }

}

I was worried i had put them in the wrong order in the build settings so I removed all levels and placed them all back in in the correct order but that didn't help. As far as i can tell they are definitely in the correct order (but as i said i am new to unity).

thanks for reading and any help would be greatly appreciated :)

 public static int currentLevel = 0;

 void OnTriggerEnter (Collider other)
 {
     if (other.transform.tag == "Goal")
     {
          manager.CompleteLevel();
     }   
 }