Gui Texture Differences

I want to be able to touch a gui image on an ios device and have it do something. I have the code:

 using UnityEngine;
 using System.Collections;
 
 public class Test: MonoBehaviour 
 {
 
     void OnMouseDown() 
     {
         Debug.Log ("Mouse Down");
         }
 
     void OnMouseUp() 
         {
         Debug.Log ("Mouse up");
     }
     
 }

In one project, I can click an empty gameobject with a guitexture attached to it, and the script works. In my main project, I can not even add a visible texture to the gui texture, so I can't use my script. If anyone knows what is happening, or an alternative, please share it. Thanks! (My code is in C#)