Changing mesh shader not working iOS

I have 2 custom shaders that we allow players to cycle through which show different topographical details on our terrain mesh. Players can toggle each shader with a button click in our UI. This has worked beautifully in our Android builds, but does not work on an iPhone. When I try and toggle the shader, it blips to pink for a split second, then goes back to what it was before. Also, it does work on an iPad just fine, which is odd. Could there be a graphics setting I am overlooking? Is this an iPhone hardware issue? Below is the script for the button to rotate shaders on our meshes.

 public class NewLayersButton : MonoBehaviour
 {
     public Material green;
     public Material collar;
     public Material rough;
     public Material[] layers;
     public string[] labels;
     public float labelTime = 1.0f;
     private Text label;
     private Image image;
     private int layerIndex = 0;
     private float labelTimer;
 
     public void Awake()
     {
         label = this.GetComponentInChildren<Text> ();
         image = this.GetComponent<Image> ();
     }
 
     public void OnGUI()
     {
         if (Time.time > labelTime + labelTimer) {
             label.text = "";
             image.enabled = true;
         }
     }
     
     public void tapFunction ()
     {
         layerIndex = (layerIndex >= layers.Length - 1)? 0 : layerIndex + 1;
         green = layers [layerIndex];
         collar = layers [layerIndex];
         rough = layers [layerIndex];
 
         label.text = labels [layerIndex];
         image.enabled = false;
 
         labelTimer = Time.time;        
     }
 }