Since some days ago, GameSpy, the service providing the server lists for the ARMA series and many other games, appears to have limited serverlist data to a maximum of 500 servers per query. Probably as a response for the huge popularity & usage these past weeks, this affects both the in-game serverbrowser as well as SIX Updater and Launcher lists.
This is the main reason why it's hard to find the same server as your friends, the server might not be listed as 1 of the only 500 servers send to you through the system.
We've put backup lists in place for the Launcher recently, these should mitigate the situation somewhat,
while a full and proper solution is prepared for our upcoming launch of Play withSIX!