OnTriggerEnter Not working

//this is the trigger


using UnityEngine;

public class endtrigger : MonoBehaviour {

    public GameManagerScript gameManager;


    void OnTriggerEnter ()
    {
        gameManager.CompleteLevel();
    }

}



//this is the game manager in a separate script

using UnityEngine;
using UnityEngine.SceneManagement;

public class GameManagerScript : MonoBehaviour {

    bool gamehasended = false;

    public float restartDelay = 1f;

    public GameObject completeLevelUI;

    public void CompleteLevel ()
    {
        completeLevelUI.SetActive(true);
        Debug.Log ("LEVELDONE");
    }

    public void endgame ()
    {
        if (gamehasended == false)
        {
            gamehasended = true;
            Debug.Log("GAME OVER");
            Invoke("Restart", restartDelay);
        }
    }

    void Restart ()
    {
        SceneManager.LoadScene(SceneManager.GetActiveScene().name);
        Debug.Log("RESTARTED");
    }

}


//my bug report (has a typo)
https://fogbugz.unity3d.com/default.asp?906597_eqkj43md65n1l36o

//brackeys forum (I was following his tutorial)

http://forum.brackeys.com/thread/ontriggerenter-not-working-2/