scene loader issue

so I made a crossfade for transition between scenes, works perfectly for loading the opening scene but wont load the next scene (no errors but wont load)

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.SceneManagement;
 
 public class LevelLoaderScript : MonoBehaviour
 {
     public Animator transition;
 
     public float transitionTime = 1f;
 
     void update()
     {
         // Press the space key to start coroutine
         if (Input.GetKeyDown(KeyCode.Space))
         {
             LoadNextLevel();
         }
     }
 
     public void LoadNextLevel()
     {
         StartCoroutine(LoadLevel(SceneManager.GetActiveScene().buildIndex + 1));
     }
 
 
     IEnumerator LoadLevel(int levelIndex)
     {
         transition.SetTrigger("Start");
 
         yield return new WaitForSeconds(transitionTime);
 
         SceneManager.LoadScene(levelIndex);
     }
     
 }