Option WHOIS recognize visitor country (and many other parameters) based on visitor IP
WHOIS option query other servers in internet, called RIPE servers, for those additional data.
To make this option work, Your server must have opened port 43 - JoomlaStats use this port to connect to RIPE servers.
Firewall or closed port 43 is the most common problem that WHOIS option does not work.
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Solution 1: (recommended)
Make sure your system (the webserver JoomlaStats is configured on) can open an outgoing connection on port 43 to whois.ripe.net
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Solution 2:
Upload data to JoomlaStats database and turn off WHOIS option in configuration
JoomlaStats is looking for developer who implement this feature.
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Solution 3:
Turn off WHOIS option in configurations (menu->JoomlaStats->Configuration->Whois option)
Remember that JoomlaStats will be unable to find country for some visitrors