I know this is an old thread but listen if you are having problems. Here's how I solved it myself with no help from Nintendo and a 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 the 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 so 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 play them again just as soon as I get a new R4 card. I also updated the firmware after nearly a decade and now my current one is outdated.
Edit August 2022: Still glad to see this continues to help people. I have read all of the comments so far. As other people pointed out, there are potential risks involved to be aware of. This is mainly potential saliva damage. So Try not to spit into it and also do not overdo it. Personally, I would take the trade-off on an old device to get it fully functional again and mine still works fine after using this trick to this day.