@Tommyg please note that the piezo buzzer has a polarity. If you look at the below image of a buzzer you will notice that the label (which you have to remove) is marked with a "+" near one of the connections.
Also, when the label has been removed, you will notice a "+" in the plastic.
This pin has to receive the positive end of the signal. It might be that you have to turn the buzzer 180 degrees to switch which pin is connected to which wire to make it work.