If you have a mobile theme set in the dashboard and you have full page caching switched on, it ignores the mobile theme and loads the cached desktop version. (If you clear the cache and load on a mobile first, then the desktop version will load that cached mobile version....I think)

I see through various posts that this is something that should be addressed in 5.7 (although I believe they are pushing for more responsive designs), it would be good to get a work around in the meantime for those older sites not built with a responsive template.

If anyone has any idea how this might be achieved, please share :)