If you are using a cache plugin like W3 Total Cache for wordpress and the website is behind a Varnish Cache Proxy, then be sure to specify the port to which Varnish responds on the server, as there are situations where port 80 is not the standard one.
Otherwise the purge action will have no effect, even if the plugin reports successfully:
Varnish server successfully purged.
If you ask yourself “what are the cases when varnish is not installed on port 80?” , the answer is simple: Varnish is not a frontend but a backend to another server/proxy that responds on port 80 or 443 (SSL), such as HAProxy!
In my case, the server configuration is as follows:
HAProxy (80,443) => Varnish (12345, 23456) => Backend IP Route (IP1, IP2) => Web Server (19191.18181)
Remember to allow the Web server via (IP1, IP2) access to one of the two ports 12345 or 23456 on the server where varnish is installed!