GUI Button

Hello everyone, I made this script, but it works only in part, when the target is detected the buttons appear, so everything is ok, but when I press the buttons to start the animations do not work, if I put a statement like "openurl "works, while the statement" animation.Play no!. did not understand where I'm wrong. Sorry for English  Thank you so much  Alessio using UnityEngine;using System.Collections; public class bottoni : MonoBehaviour, ITrackableEventHandler {   private TrackableBehaviour mTrackableBehaviour;         private bool mShowGUIButton = false;            void Start () {        mTrackableBehaviour = GetComponent<TrackableBehaviour>();        if (mTrackableBehaviour)        {            mTrackableBehaviour.RegisterTrackableEventHandler(this);        }    }         public void OnTrackableStateChanged(                                    TrackableBehaviour.Status previousStatus,                                    TrackableBehaviour.Status newStatus)    {        if (newStatus == TrackableBehaviour.Status.DETECTED ||            newStatus == TrackableBehaviour.Status.TRACKED)        {            mShowGUIButton = true;        }        else        {            mShowGUIButton = false;        }    } void OnGUI () {        if (mShowGUIButton) {  if (GUI.Button (new Rect (10,10,110,50), "cella")) {animation.Play("cella"); }if (GUI.Button (new Rect (10,70,110,50), "orologio")) {animation.Play("orologio");        }if (GUI.Button (new Rect (10,130,110,50), "portale")) {animation.Play("portale");            }if (GUI.Button (new Rect (10,190,110,50), "EXIT")) {Application.Quit(); }  } } }