Lots of electronic devices have problems with headphone jacks that don't work well or work only intermittently. The iPhone is getting a reputation for this but it can happen to any sort of portable device like a cell phone or a music player since they get carried around a lot and exposed to a lot of dirt and other cruft. If you have a headphone jack that is not working reliably, something to try is compressed air. You can find cans of compressed air at any place that sells camera equipment. Just spray the air directly into the jack. If there is any dirt or some other foreign object in there that is messing up the connection, the compressed air will usually knock it out. Of course if your problem is related to the wiring or something like that, this won't do anything. But this is a cheap fix to try so it's usually worth a shot.