I repaired my Plus keyboard about six yers ago, and made a toool to pull the keys off and two metal strips to ease the switch out when de-soldering the pins.

After dissembling the switch, I inserted a small piece of cardboard (coloured purple in the second photo) between the switch board and the switch body as I found this makes them work again.

The switches stop working due to a physical lack of contact at the point shown in the third photo.