How can I make the terrain detail shading more advanced?

I've already got the shader I want working as a normal shader. But when I try to override the "WavingGrass" shader with my changes it turns black, complains about a lack of normals and has a flat specular term. No matter what I do to the shader this persists. Something in the terrain is overriding my custom lighting shader. How can I stop it from doing this?

I'm not keen on implementing my own terrain system just to get around this, and I don't like the flat shading of standard detail rendering.