It's happened to mine. The contacts inside could just be dirty. There are some Youtube videos of people cleaning it out with water and a Q-tip. I just had that problem but 15 minutes ago and found a really quick and easy fix that worked for me. Take your lips and form a perfect seal around the L button and then blow a few times. It blows the dust out from inside where you can't normally reach (damn tri-wing screws). It's kind of like the theory behind blowing in old school NES games. I swear this is no joke, it worked for me surprisingly (least technically involved troubleshooting I've ever had to do...I was SO surprised). I hope this helps!