In reply to panwei1988:
There might be several reasons for this message. Did you register my_scoreboard1 with the factory?
Where are you calling the override?
To debug this situation you can call
factory.print();
This gives you all overrides regsitered with the factory.
You should call this from the test you aare running, directly before the run_phase. I do this normally in the start_of_simulation_phase.