One of my sites is behaving weird, all other sites are fine. I’m trying to pinpoint what the difference is but all sites I host are using (more or less) the same config…
If I go to this site: http://web-sniffer.net/ and check this site: http://www.ict-consult.co.za/ (hosted on my server) I see this directive:
Date: Thu, 01 Jan 1970 00:00:00 GMT
If I check another site, i.e. http://www.intramed-distribution.co.za it shows:
Date: Sun, 23 Mar 2014 19:12:32 GMT
Some also have these headers:
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
And on top of that, apparently only one site shows this header:
Fastcgi-Cache: HIT
All the others always say: BYPASS 
Only one of my sites uses ngx_pagespeed as I activate it per site.
Not even sure where to start…
What do you suggest? Shall I install 2 new WordPress instances that are both running straight through my server and not through any other CDN, i.e .Cloudflare, then only activate your NGINX Helper plugin and nothing else and see if I get similar results? All sites use the same basic nginx.conf + the other snippets I manually give the vhosts, but for testing purposes I can keep those to the same basics.