Took me forever to figure out why Previews worked at times, an other time not. Turns out that when I'm not signed in, Page Preview works. When I notice I'm not signed in, an do so, the Page Previews no longer work.
No changes in browser settings, extensions, etc.
Page previews were never mass enabled for pre existing users. So if you never manually enabled it, then you still need to enable in your Preferences. They are in the appearance section and then: Enable page previews.
Another reason can be that you have some sort of competing gadget or user script installed. Navigation popups for instance disables Page Previews.
