Problem : No matching host


i am a new user of this great tool named nagios, and i have a little problem with it :

I have to monitor servers in different country (belgium, ireland, luxembourg), I made 1 config file for each country, and when i look in the web interface to see the results, theres is only 1 country that is working correctly : the 2 others work for 5 sec then the servers dissapears and i have a message : No matching host, 5 min after it’s working correctly again, and it keep on doing this every 5 sec. …

if you have an idea on how to resolve this please let me know

thank you in advance .