I printed this a while ago, but I didn't have conductive PLA.
So I printed the center piece with normal PLA. Of course this wouldn't work.
To fix it I cut out a piece of aluminium tape, put it on the bottom of the piece and wrapped it around and onto the thumbpad.
Just coating the underside does not work, it has to be in contact with your finger somehow. Not sure why, probably for conductivity reason. I am not an expert on touchpad technology.
I also used a tiny piece of blu-tack to keep the center piece from flying off when I release the pad and bent the plastic very slightly so that it rests 1 mm above the thumb pad when not touched.