This probably won't help, but it's worth a try. And that's cleaning them, preferably with 90%+ isopropyl alcohol.
If that doesn't help, you will probably need to replace the whole joy-con, I don't really know how it looks in the Quest Controller but I'm 99.9% sure that it's using the same design as everyone else. So you may need to figure out how to get it apart yourself.
The problem is in the design where after some time the surface in the joy-con that the signal goes through is worn off and loses contact.
***Hope this helped! :D***