Vuforia ImageTarget Disabled when game started cannot track object

about Vuforia imageTarget : if imageTarget is diabled when game started
I attach a function to setActive the imageTarget object after 5seconds, it cannot track anymore,why is that?
the imageTarget is set active and OnTrackableStateChanged is called once, from state UNKNOWN to NOT_FOUND , it cannot be called second time even if i put the targeted image in front of the camera.
here is my code:

public GameObject object1;

void Start()
{
if(object1)
object1.SetActive(false);
StartCoroutine(wait());
}

IEnumerator wait()
{

yield return new WaitForSeconds(3);
if (object1)
object1.SetActive(true);
}

and the ImageTarget object is attached to object1 in my script.
It is deactivated when started.But after it is activated, it cannot track object


I add log lines in function OnTrackableStateChanged

public void OnTrackableStateChanged(
TrackableBehaviour.Status previousStatus,
TrackableBehaviour.Status newStatus)
{
Debug.LogError("Trackable " + mTrackableBehaviour.TrackableName + " changed");
if (newStatus == TrackableBehaviour.Status.DETECTED ||
newStatus == TrackableBehaviour.Status.TRACKED ||
newStatus == TrackableBehaviour.Status.EXTENDED_TRACKED)
{
Debug.LogError("Trackable " + mTrackableBehaviour.TrackableName + " found");
OnTrackingFound();
}
else if (previousStatus == TrackableBehaviour.Status.TRACKED &&
newStatus == TrackableBehaviour.Status.NOT_FOUND)
{
Debug.LogError("Trackable " + mTrackableBehaviour.TrackableName + " lost");
OnTrackingLost();
}
else
{
// For combo of previousStatus=UNKNOWN + newStatus=UNKNOWN|NOT_FOUND
// Vuforia is starting, but tracking has not been lost or found yet
// Call OnTrackingLost() to hide the augmentations
Debug.LogError("Trackable " + mTrackableBehaviour.TrackableName + " unkonwn");
OnTrackingLost();
}
}

the Debug.LogError("Trackable " + mTrackableBehaviour.TrackableName + " changed");
is only called once

 

I have the same issue!!!