1°) Ensure the config files are all included (starting with main nginx..conf
files, then following all 'include' logic). The 'sites-available' logic is
due to some distributions packaging and is not there in the official nginx
one.
Usually this is OK, but checking cannot hurt. :o)

2°) Ensure your configuration is *loaded*, ie the syntax is correct and
nginx is actually applying it.
- use nginx -t to check the available config does not contain errors
- send SIGHUP to nginx master to reload the configuration (and check logs
for messages regarding it)
.... or use the probably available 'reload' command from your system
management script which does both steps, knowing what you are doing

3°) Ensure the reply you see in your client is the one sent by the server
(ie take care about any intermediate cache).
One way of effectively doing that is monitoring the access.log file (which
exists in the default nginx configuration
http://trac.nginx.org/nginx/browser/nginx/conf/nginx.conf, though
commented).
Another (complementary) way of doing the same is using cURL on the
command-line, querying your server on the right interface

If you are still experiencing difficulties, please provide details about
your problem and your attempts (and their result aswell) to solve it.
---
*B. R.*
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