HFSS MS sims with thin traces/ground on lossy substrates?

Any suggestions on using HFSS 9.2 to simulate thin vs thick MS signal traces? Also MS lines with thin ground plane on top of lossy (Si) substrates?
When I vary the thickness or roughness of my signal line, or vary the ground plane thickness or roughness (on top of lossy silicon), S21 behavior stays exactly the same (usual decay with frequency due to gold skin effect loss)...

Hi,
could you please attach the hfss file so i can see what the problem is?