6 different diffuse textures with 6 of it's specs and 6 normals 18 textures total for the entire material? But there isn't an error in the material editor. Should that be ok? It's like a component can only have 3-4 textures painted per square anymore than that will crash.
Generate your specular in the mat editor instead, use mergemaps, or use the blue channel from the normal map for specular, and you should be able to retain your 6 layers with spec and normal mapping probably, if the landscape wants to accept all that in its material.