I'm trying to create a shader that uses separated alpha, specular and normal maps along with a color texture, but I'm having some strange problems with it and I can't figure out what's wrong.

Here's my shader:

Shader "Custom/DiffuseAlphaNormalSpecular" {
Properties {
    _MainTex ("Diffuse", 2D) = "white" {}
    _AlphaMap ("Alpha", 2D) = "white" {}
    _BumpMap ("Normalmap", 2D) = "bump" {}
    _SpecColor ("Specular Color", Color) = (0.5, 0.5, 0.5, 1)
    _SpecularMap ("Specular", 2D) = "white" {}
    _Shininess ("Shininess", Range (0.01, 1)) = 0.5
}
SubShader {

    Tags {"Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent"}
    LOD 300

    CGPROGRAM
    #pragma surface surf BlinnPhong alpha decal:blend

    sampler2D _MainTex;
    sampler2D _AlphaMap;
    sampler2D _BumpMap;
    sampler2D _SpecularMap;
    half _Shininess;

    struct Input {
        float2 uv_MainTex;
        float2 uv_AlphaMap;
       float2 uv_SpecularMap;
       float2 uv_BumpMap;
    };

    void surf (Input IN, inout SurfaceOutput o) {
        half4 d = tex2D (_MainTex, IN.uv_MainTex);
        half4 a = tex2D (_AlphaMap, IN.uv_AlphaMap);
       half4 b = tex2D (_BumpMap, IN.uv_BumpMap);
       half4 s = tex2D (_SpecularMap, IN.uv_SpecularMap);

        o.Albedo = d.rgb;
        o.Alpha = Luminance(a.rgb);
       o.Normal = UnpackNormal(b);
       o.Specular = _Shininess;
       o.Gloss = s.rgb * o.Alpha;;
    }
    ENDCG
} 
FallBack "Transparent/Diffuse"  
}

and here is the problem: