I was scratching my head for hours in troubleshooting this issue. My WordPress website which was caching fine with FastCGI, suddenly stopped caching.
I couldn’t see any HIT or files and folder being created inside the cache folder. And there is no error either.
After so many struggle, I found a blog post with the exact issue.
You simply need to add this to your configurations
# ignore headers to let Nginx control its own cache fastcgi_ignore_headers Cache-Control Expires;
This will ignore the caching header and let Nginx cache the response.