The reason why the switch will have stopped working is clear in the picture

The switch has lost the lumps which contact the pin in turn pushing the ribbon switches

Without these nothing happens.

Hopefully this guide has helped as I could not find any information on the internet. I ended up buying a 2nd hand midframe to experiment with.

As a result I must be in a small minority of chrome mate 20 pro's with a purple volume button :-) !