I know this is an old thread but listen if you are having problems. Here's is how i solved it myself with no help from nintendo and super easy method.
My bumpers have not worked for over 5 years. both L & R. when I press R, it sounds like it has a broken spring while L sounds normal but also won't work.
Today, I tried to get them working(With Success) by first, mashing them really hard and fast and pushing down on them. After , I tried blowing in the side really hard just with my mouth. No taking apart. Then I kept mashing them again and on the home screen to test to see if camera comes on when pressing L or R. All of a sudden, after blowing, it started to work every time. R still sounds and feels odd but works. I just have to press them harder than normal though but they work fine now!! I was soo happy after they started working. it has been years. I was unable to play soo many old games as they required the bumpers but now I can paly them again jsut as soon as I get a new R4 card as I also updated the firmware after nearly a decade and now my current one is outdated.