Is my book not working because of a shader problem?

Hi everybody! I'm currently developing an application where part of it is a virtual book and a pageflip animation. When the user clicks and drags the mouse to the left the page is turned.

I'm having a problem with showing the pages and I really need your help! The setup is branching off this project (specifically what Bérenger Mantoue answered in this thread) and looks like this:

There is a camera for every page. There are other objects such as stencils and masks.

The above mentioned objects are associated with a layer like this:

Object – Layer

• Page 1 – A

• Page 2 – B

• Page 3 – C

• Page 4 – D

• Page 5 – E

• Page 1 – F

• Mask1 – mLayer

• Mask2 – mLayer2

• Mask1's stencil – Stencil

• Mask2's stencil – Stencil2

• Page 4's stencil – D

• Page 6's stencil - F

Each camera has a depth and one or more layers to display:

• Camera - Layer(s) - Depth

• Main cam - A - (-1)

• cam 2 - B, mLayer - 1

• cam 3 - C, Stencil - 2

• cam 4 - D, mLayer2 - 0

• cam 5 - E, Stencil2 - 3

• cam 6 - F - (-2)

Besides that, each page also has a shader:

Page - Shader

• Page1 - Pageunlit

• Page2 - Pageunlit

• Page3 - Overmask

• Page4 - Overmask

• Page5 - Overmask

• Page6 - Overmask

The role of Mask1 is to uncover page2 so that the 4th page is shown as the 3rd page turns. When the pageturn is done page 3 and page 4 are then fully viewable.

The role of Mask2 is to uncover page4 so that the 6th page is shown as the 5th page turns. When the pageturn is done page 5 and page 6 should be viewable but they are not. For some reason, when I make the second page turn (which should reveal page 5 and 6), page 4 is not masked like page 2 is – even though they have the same setup.

My guess is that this behaviour is caused by the shaders and the way it interprets Ztest, but I can't figure out what is wrong. Here is the code of the shaders:

---

Overmask.shader:

 Shader "Livre/Over Mask" 
 {
     Properties {
         _Color ("Main Color", Color) = (1,1,1,1)
         _MainTex ("Base (RGB)", 2D) = "white" {}
     }
     
     SubShader {
         Tags { "Queue" = "Geometry-9" }
         Lighting Off
         ZTest GEqual
         LOD 200
     
         CGPROGRAM
         #pragma surface surf Lambert
         
         sampler2D _MainTex;
         fixed4 _Color;
         
         struct Input {
             float2 uv_MainTex;
         };
         
         void surf (Input IN, inout SurfaceOutput o) {
             fixed4 c = tex2D(_MainTex, IN.uv_MainTex) * _Color;
             o.Albedo = c.rgb;
             o.Alpha = c.a;
         }
         ENDCG
     }
     
     Fallback "VertexLit"
 }

---

PageUnlit.shader:

 Shader "Livre/Page Unlit"
 {
     Properties 
     {
         _Color ("Main Color", Color) = (1,1,1,1)
         _MainTex ("Base (RGB)", 2D) = "white" {}
     }
     SubShader 
     {
         Tags { "RenderType"="Opaque" }
         Lighting Off
         LOD 200
 
         CGPROGRAM
         #pragma surface surf Lambert
 
         sampler2D _MainTex;
         fixed4 _Color;
 
         struct Input {
             float2 uv_MainTex;
         };
 
         void surf (Input IN, inout SurfaceOutput o) {
             fixed4 c = tex2D(_MainTex, IN.uv_MainTex) * _Color;
             o.Albedo = c.rgb;
             o.Alpha = c.a;
         }
         ENDCG
     }
 
     Fallback "VertexLit"
 }

So, what I'm looking for is answers to the following:

1. Can anyone help me understand why page 4 is not getting masked like page 2 is?

2. Sub-question: Is the 'ZTest' in the Overmask shader affected by the depth of the camera or Z-value in world coordinates? Or another value that I'm not considering?