They are only supposed to open when the airflow in the venturi is moving fast enough. Your engine has to be consuming enough air to do that. Stomp on it at 45-55 mph and take it to passing speed.
The signal can be improved by using a tube into the venturi like Holley did at least into the '70s for the 700 cfm and larger carbs.
see here: Vacuum Secondary Diaphragm Signal Tube
