Problem with sprite materials/shaders inside a mask

Hi everyone, I have a problem with having a sprite with material inside a mask, this is the shader that I use.

 Shader "Sprites/DefaultColorFlash"
 {
     Properties
     {
         [PerRendererData] _MainTex("Sprite Texture", 2D) = "white" {}
     _Color("Tint", Color) = (1,1,1,1)
         _FlashColor("Flash Color", Color) = (1,1,1,1)
         _FlashAmount("Flash Amount",Range(0.0,1.0)) = 0.0
         [MaterialToggle] PixelSnap("Pixel snap", Float) = 0
 
         _StencilComp("Stencil Comparison", Float) = 8
         _Stencil("Stencil ID", Float) = 0
         _StencilOp("Stencil Operation", Float) = 0
         _StencilWriteMask("Stencil Write Mask", Float) = 255
         _StencilReadMask("Stencil Read Mask", Float) = 255
 
         _ColorMask("Color Mask", Float) = 15
 
         [Toggle(UNITY_UI_ALPHACLIP)] _UseUIAlphaClip("Use Alpha Clip", Float) = 0 
     }
 
         SubShader
     {
         Tags
     {
         "Queue" = "Transparent"
         "IgnoreProjector" = "True"
         "RenderType" = "Transparent"
         "PreviewType" = "Plane"
         "CanUseSpriteAtlas" = "True"
     }
         Stencil
     {
         Ref[_Stencil]
         Comp[_StencilComp]
         Pass[_StencilOp]
         ReadMask[_StencilReadMask]
         WriteMask[_StencilWriteMask]
     }
 
         Cull Off
         Lighting Off
         ZWrite Off
         ZTest[unity_GUIZTestMode]
         Blend One OneMinusSrcAlpha
         ColorMask[_ColorMask]
 
         Pass
     {
         CGPROGRAM
 #pragma vertex vert
 #pragma fragment frag
 #pragma multi_compile DUMMY PIXELSNAP_ON
 #include "UnityCG.cginc"
 
         struct appdata_t
     {
         float4 vertex   : POSITION;
         float4 color    : COLOR;
         float2 texcoord : TEXCOORD0;
     };
 
     struct v2f
     {
         float4 vertex   : SV_POSITION;
         fixed4 color : COLOR;
         half2 texcoord  : TEXCOORD0;
     };
 
     fixed4 _Color;
     fixed4 _FlashColor;
     float _FlashAmount;
 
     v2f vert(appdata_t IN)
     {
         v2f OUT;
         OUT.vertex = mul(UNITY_MATRIX_MVP, IN.vertex);
         OUT.texcoord = IN.texcoord;
         OUT.color = IN.color * _Color;
 #ifdef PIXELSNAP_ON
         OUT.vertex = UnityPixelSnap(OUT.vertex);
 #endif
 
         return OUT;
     }
 
     sampler2D _MainTex;
 
     fixed4 frag(v2f IN) : COLOR
     {
         fixed4 c = tex2D(_MainTex, IN.texcoord) * IN.color;
     c.rgb = lerp(c.rgb,_FlashColor.rgb,_FlashAmount);
     c.rgb *= c.a;
 
     return c;
     }
         ENDCG
     }
     }
 }

What I do with it is basically change the _FlashAmount property.

In the image the left button is inside a mask and the right one is not. As you can see the left button doesn't seems to be affected by the shader. The problem must be in the shader. Anyone know how to do this? I need the mask to crop the list where the button is placed.