I had this problem when I dropped my 3ds. what I did was (carefully) disassemble the button itself and put a very very very small piece of thin cardbordin between the black mechanical part of the button and the plastic that presses against it. Fixed it completely. I would have taken pictures or video, but all pieces are extremely small and very difficult to deal with. I don't blame ifixit for not making a guide on the buttons lol.
I fixed it myself. Since I could not find any guides on the innards of the shoulder buttons, I decided to see what's up with it myself. I opened them both up, discovered that the nub that presses down the inner button wasn't reaching the inner button for some odd reason. So I cut a little tiny piece off of a birthday card and put it underneath the inner button to push it up. Now when I press the R button, I have to press down pretty hard for it to react, but at least it works! Also, warning to those who want to open the shoulder buttons up, the springs are VERY bouncy and there is one nut per shoulder button that can fall out very easily so be careful! It is a PAIN to put the shoulder buttons back in.
This is a necro by a long shot, but I'm afraid my macbook pro late 2011 is having the same problem. For several years I have been baffled by these small balls coming out of nowhere, but now everything makes sense that I know it's from my computer. I have no idea how to fix this and I have never seen these balls before inside my computer, even though it is obvious to me now, that they are coming from there.