This happens to me all the time. Sometimes you get a bit of grime over the contacts in the remote so that the conductive rubber on the underside of the buttons doesn't connect the traces of the switch. If it's just grime, cleaning the rubber keypad with some isopropyl alcohol should do the trick. However, as mentioned, if the rubber itself wears out, you can either buy some conductive paint (which will wear off eventually as well) or just get a universal remote (I have used a bunch of these in the past).
Regards,
Matt