What makes "Reader mode" available?

Some of our website users say that Reader Mode was avalable before but now they can’t see it. I had never tested it but now I only can see the install-PWA button where the “Reader Mode” buton should be.
I have tested it in other sites and the button works. Is there any kind of metadata or condition to make it available again?
Thanks

Hello,

Please create a support request with details and log in the support chennel.

Thank you