Make sure your Subwoofer is connected via the "Sub-out" port behind the keyboard and not the L+R AUX port. And also make sure your keyboard's mixer's master settings is set to "With Subwoofer". Also note that when the subwoofer is connected via "Sub-out" the subwoofer's volume is completely independent i.e. your keyboard's volume doesn't affect the subwoofer's volume, instead you need to separately set the subwoofer volume using its onboard volume knob.
Regards,
Chris.