Try changing the variable SpeedA to 3
(I have not used this shield)
I think it requires a signal on pin3 to set the speed of the motor. Currently it is not sending anything, so the motor is not moving. If you touch pin3, static electricity or your bodies capacitence or something induces a slight current on pin3, which makes the motor move.
you also need to the actually send the signal. probably analogWrite(speedA, 100); or something like that. (the value after speedA should be something between 0 and 255)
Does that help?
Search the forums, there is more info floating around.
p.