Then it's definitely not piezo-electric

Other possible reasons for it not working: wrong impedance (on older devices it would just play quietly, new devices may have built-in protection that shuts it off completely), too little power, bad soldering (sorry but it happens to the best of us sometimes).

It may also be that the device does not like it if only one audio channel is loaded. Some devices can be persuaded to play mono through a stereo output if you connect the speaker to the left channel. That would be contacts 1 and 3 in the picture (picture courtesy of Wikipedia). Others can be tricked by shorting L+R (contacts 1 and 2), but that can cause problems if the stereo signal is way off the balance.