On 12/06/2019 19:16, NY wrote:
What is the best way of securing the captive nuts on an electrical
backing box for a light switch or mains socket?

I always find that if I unscrew the front screws (eg so I can paint
behind the switch, or to replace a broken switch with a new one) I have
great difficulty in getting the screws to mate with the captive nuts
because they are often not fixed to the backing box but instead slide
backwards so the screw can never mate with the nut.

Even if the screw does mate, it seems not to be secure and once I
tighten it beyond a certain point it starts to jump thread instead of
applying enough force to hold the switch against the wall.

Use longer screws. Don't overtighten.

Bill