Level dont loaded, after enter a Trigger BoxCollider

Hello, i want, that the game loads the next level, when my player enter a Triggerbox. But it doesnt work. When i make a button for load the next level it works? The player has the right tag. I have no idea. The script is attached to the box collider of the levelend.

My code:

 using UnityEngine;
 using System.Collections;
 
 public class LoadNextLevel : MonoBehaviour {
     public string levelName ="";
     public string tag = "";
 
     // Use this for initialization
     void Start () 
     {
     
     }
     
     // Update is called once per frame
     void Update () 
     {
     
     }
     
     
     void OnTrigerEnter(Collider other)
     {
         if(other.gameObject.tag == tag)
         {
             Application.LoadLevel(levelName);
             Debug.Log("NextLevel");
         }
     }
     
     void OnGUI()
     {
         if(GUI.Button(new Rect(300,300,100,100),"NextLvl"))
         {
         
             Application.LoadLevel(levelName);
         }
     }
 }