disable ImageTarget at start then activate it, imageTarget dosent work.

I disabled ImageTarget at first, and then start a coroutine to set it active, but it doesnot track images,why is that ?
if i set it inactive at runtime then set it active, it can work well
here is my code:

//This is the field of view that the Camera has
public GameObject object1;

void Start()
}IEnumerator wait()
    yield return new WaitForSeconds(3);

i attach imageTarget to object1
but it didnt work , onTrackingLost is only called once,and onTrackingFound is never be called

Did you find an answer to this?