SteamVR_Controller.GetPress , GetPressDown, GetPressUp always returns false !

 using UnityEngine;
 using System.Collections;
 using Valve.VR;
 
 public class WandController : MonoBehaviour {
     private EVRButtonId triggerButton = EVRButtonId.k_EButton_SteamVR_Trigger;  
     public bool triggerButtonDown = false;
     public bool triggerButtonUp = false;
     public bool triggerButtonPressed = false;
 
     private EVRButtonId gripButton = EVRButtonId.k_EButton_Grip;               
     public bool gripButtonDown = false;
     public bool gripButtonUp = false;
     public bool gripButtonPressed = false;
 
     private SteamVR_Controller.Device controller { get { return SteamVR_Controller.Input((int) trackedObj.index); } } 
     private SteamVR_TrackedObject trackedObj;
 
     void Start () {
         trackedObj = GetComponent<SteamVR_TrackedObject>();
         Debug.Log("trackedObj: "+trackedObj);                   //trackedObj is never null
     }
     
     // Update is called once per frame
     void Update () {
         if (controller == null)
         {
             Debug.Log("Controler Object ist null");             // controller object is never null
             return;
         }
 
 
         gripButtonDown = controller.GetPressDown(gripButton);   //is always false, also when pressing gripButton
         gripButtonUp = controller.GetPressUp(gripButton);
         gripButtonPressed = controller.GetPress(gripButton);
 
         
 
         triggerButtonDown = controller.GetPressDown(triggerButton);
         triggerButtonUp = controller.GetPressUp(triggerButton);
         triggerButtonPressed = controller.GetPress(triggerButton);
         Debug.Log("triggerButton: "+ controller.GetPress(triggerButton));  //is always false, also when pressing triggerButton
         Debug.Log("triggerButtonDown: " + triggerButtonDown);              //is always false, also when pressing triggerButton
 
 
 //content of these if-statements never get activated/run -> also when pressing all Buttons permanently ! 
         if (gripButtonDown)
         {
             Debug.Log("Grip Button was just pressed");
         }
 
 
         if (gripButtonUp)
         {
             Debug.Log("Grip Button was just unpressed");
         }
 
 
         if (triggerButtonDown)
         {
             Debug.Log("Trigger Button was just pressed");
         }
 
 
         if (triggerButtonUp)
         {
             Debug.Log("Trigger Button was just unpressed");
         }
 
     }
 }

This script is attached to 'Controller (left)'.

Why do the methods 'controller.GetPressUp(gripButton); controller.GetPressDown(gripButton); controller.GetPressDown(triggerButton);' etc. always return false ; even though I'm pressing all the Buttons(Trigger, Grip, and all other buttons) on the Vive controller (left) ?

Thanks