Really not clear what's going on here... What is your "colour to grayscale" node being used for? It's not connected to anything. When you say "subdivide mesh", are you referring to the option in the Rock Object? You're saying that when Subdivide Mesh is enabled, Reflectivity doesn't work? But clearly you're doing some kind of displacement as well, so does it happen without that? Try it with the most basic setup you can.
- Oshyan