The iPad headphone socket is a very tight fit - you need to push it in really firmly. If it's not pushed in all the way - a problem if you have the Apple case where the cut-outs may obstruct the headphone plug - then you'll either get no sound at all or just one 'side' - i.e. left or right...try pushing the socket in very hard.
If that doesn't work, please don't hesitate to get back to us....
Tim
Scotland