capacitive sensor and 9volt power

hello i got the arduino start kit (arduino uno r3)

i build the kit #13 capacity sensor

the kit work when powered with usb cable

but the kit donot work when powered with a 9v battery (jack connection)

why ?

please help me

Do a Google search for "arduino cap sense ground" and you will see why the removal of ground (through the USB connection) makes a difference.