Lighting induced seams

I have created a terrain that is made up of several tiles and I am currently trying to add lighting to this. I have run into a problem that could be with my shader but is causing the tiles, whose vertices and normals should agree at the edges, to have seams and lighting artifacts. Here are two examples: With my shader

With default diffuse lit

This is what I am using to calculate the normal direction

 float3 normalDirection = normalize(float3(
                mul(float4(input.normal, 0.0), modelMatrixInverse)));

And here is how I get light direction:

 float3 lightDirection = normalize(
                float3(_WorldSpaceLightPos0));

And calculating the diffuse reflection with this bit:

 float3 diffuseReflection = 
                float3(_LightColor0) * float3(_Color)
                * max(0.0, dot(normalDirection, lightDirection));

The only thing that I think would cause this is the normals not being calculated correctly. The surface in these images is seamless as the vertices at the edges have the same world position.

EDIT: In the editor I have highlighted the 4 planes pictured here, as you can see their edge vertices match up with each other.