Official Smart case does lock screen

hello

i have an official apple iPad smart case I bought years ago that I stopped using on the same iPad and have recently gone back to using it on the same iPad to find it no longer locks/unlocks the screen.

ive read lots of claims sensors have changed etc. Not on mine because it use to work now it doesn't. All that's changed is software.

there is no option to turn lock case on

what am I doing wrong?