AR-Foundation prefab spawned with offset

Good Morning, with ARfoundation and Image tracking when instantiate a prefab, the model not appears at marker position, but have some offset and looks weird..where is the problem? thanks
the code:

public class Game_Interactions : MonoBehaviour
{
[Header("The length of this list must match the number of images in Reference Image Library")]
[SerializeField]
private GameObject[] placedprefs;
public Text testo;
private GameObject test;


private ARTrackedImageManager arTrackedImageManager;
void Awake()
{
arTrackedImageManager =FindObjectOfType<ARTrackedImageManager>();
}
private void OnEnable()
{
arTrackedImageManager.trackedImagesChanged += OnImageChanged;
}
private void OnDisable()
{
arTrackedImageManager.trackedImagesChanged -= OnImageChanged;
}
public void OnImageChanged(ARTrackedImagesChangedEventArgs _args)
{
foreach(ARTrackedImage track in _args.added)
{
placeobject(track);
}

}
public void placeobject(ARTrackedImage tracker)
{

if (tracker.referenceImage.name == "civita_attuale")
{
testo.text = tracker.referenceImage.name;
test = Instantiate(placedprefs[0], tracker.transform.position
, tracker.transform.rotation) as GameObject;
}
}

}