All you gotta do is get a safety pin, stick it in and CAREFULLY scrape out the dust. You probably can't see the dust, but it is definitely there. I had this issue just last night and I had enough of it so I jabbed the caked in dust In my headphone jack and scraped it out. It does take about 15 minutes, but its well worth it. If you do go ahead with this be really careful not to damage the little gold dots in the jack. Screw up a tissue and make sure it's really tightly wound so its thin. Use this tissue to dust the inside of the jack. Keep doing this until the headphones click in properly. Listen to a song to see if both sides of the track are now playing.