Always Online never works (502 error)

I’ve got multiple Cloudflare Pro websites setup - and have a setup where a 504 error is being thrown. Always online does not activate. I’ve been in touch with support and they simply point me to their page over here - which has been read - https://support.cloudflare.com/hc/en-us/articles/202238800 - but nothing obvious for my setup.

I have seen always online in some of my Apache logs - but not all - and these are Pro websites.

Has anybody else done any research into this to discover whether or not the Always Online feature works reliably all the time - I’m struggling to see where things are going amiss.