HLSL Point Light shader causing tiles to be shaded separately - graphics

Question

I've recently started learning HLSL after deciding that I wanted better lighting than what BasicEffect offered. After going through many tutorials, I found this and decided to learn from it:
http://brooknovak.wordpress.com/2008/11/13/hlsl-per-pixel-point-light-using-phong-blinn-lighting-model/
It seems that the shader above doesn't work very well in my game though, because my game uses a tile based approach, which means multiple models in a grid-like formation.
What happens is that each of my tiles gets shaded separately from the others. Please see this image for a visual reference:
http://i.imgur.com/1Sfi2.png
I understand that this is because each tile has it's own model and the shader doesn't take into account other models as it's executing on the meshes of a model.
Now, for the question. How does one go about to shade all the tiles together? I understand that I may have to write a shader from scratch to accomplish this, but if anyone could give me some tips on how to achieve the effect I want, I'd really appreciate it.
It's late so there's a possibility that I've forgotten something. If you need more information, please tell me and I'll add it.
Thanks, Merigrim
EDIT:
Here is my code for drawing a model:
public void DrawModel(Model model, Matrix modelTransform, Matrix[] absoluteBoneTransforms, Vector3 color, float alpha = 1.0f, Texture2D texture = null)
{
foreach (EffectPass pass in effect.CurrentTechnique.Passes)
{
foreach (ModelMesh mesh in model.Meshes)
{
foreach (ModelMeshPart part in mesh.MeshParts)
{
part.Effect = effect;
Matrix world = absoluteBoneTransforms[mesh.ParentBone.Index] * modelTransform;
effect.Parameters["World"].SetValue(absoluteBoneTransforms[mesh.ParentBone.Index] * modelTransform);
effect.Parameters["View"].SetValue(camera.view);
effect.Parameters["Projection"].SetValue(camera.projection);
effect.Parameters["CameraPos"].SetValue(camera.cameraPosition);
Vector3 lookAt = camera.cameraPosition + camera.cameraDirection;
effect.Parameters["LightPosition"].SetValue(new Vector3(lookAt.X, 1.0f, lookAt.Z - 5.0f));
effect.Parameters["LightDiffuseColor"].SetValue(new Vector3(0.45f, 0.45f, 0.45f));
effect.Parameters["LightSpecularColor"].SetValue(new Vector3(0.45f, 0.45f, 0.45f));
effect.Parameters["LightDistanceSquared"].SetValue(40.0f);
effect.Parameters["DiffuseColor"].SetValue(color);
effect.Parameters["AmbientLightColor"].SetValue(Color.Black.ToVector3());
effect.Parameters["EmissiveColor"].SetValue(Color.White.ToVector3());
effect.Parameters["SpecularColor"].SetValue(Color.White.ToVector3());
effect.Parameters["SpecularPower"].SetValue(10.0f);
if (texture != null)
{
effect.Parameters["DiffuseTexture"].SetValue(texture);
}
mesh.Draw();
}
}
pass.Apply();
}
}

Answer 1

It seems that the normals were the villain this time around. After correcting the normals in Blender, everything seems to work now.
I want to thank meds and Andrew Russell. Without your help I wouldn't have figured it out!
So now I know, when you have problems with your lighting, always check the normals first.