In your iMID you can switch on/off your idle stop warning messages. This will show why the car chooses not to stop the engine when you believe all conditions are right. If its not displaying any message, it may not know that it ought to stop the engine, for example if one of the clutch pedal position sensors (micro switch) was stuck/broken. If you're just getting an idle stop unavailable symbol with no other reason, I imagine your friendly dealer should be able to diagnose the problem with their fancy kit!!