problem with " WaitForSeconds"

I write this code:

Code (CSharp):

1. public class Itempick: MonoBehaviour
2.  
3. {
4.  
5.   public GameObject a;
6.  
7.  
8.  
9.   void Start()
10.  
11.   {
12.  
13.    
14.  
15.   }
16.  
17.  
18.  
19.   void OnTriggerEnter(Collider other)
20.  
21.   {
22.  
23.     if(other.CompareTag("Player"))
24.  
25.     {
26.  
27.       StartCoroutine (wait ());
28.  
29.     }
30.  
31. }
32.  
33.  
34.  
35. IEnumerator wait(){
36.  
37. a.gameObject.SetActive (true);
38.  
39.       yield return new WaitForSeconds(3) ;
40.  
41.       a.gameObject.SetActive (false);
42.  
43. }
44.  
45.  
46.  
47. }

(its exactly the same as example in unity document)!

But only "a.gameObject.SetActive (true);" works and the other one not!

 

NO - this script is attached to other GameObject (Coin) and in Inspector I choose one GameObject (Text) which I want to active and detective.