If it started working again, it feels less likely that the switches have broken (plus the fact that they look fine). That’s good news, I think
Yes - dirt or dust is plausible. If I remember correctly, the parts creating contact sit below the white plastic part, inside the switch housing. So maybe try spraying some electronics/contact cleaner inside?
Edit: here’s a thread going into detail on the switch construction.
…I miss shigginpit …
…