Keep in mind it may take time for the GameSpy master server to recieve and process an update from gameworld servers on the net. If your friends join just after Gamespy is updated by the server they're playing on, the Buddy List will not have an accurate location of where they are. It may take time for the "heart beat" ping to cycle around to that server for another check on who's actually playing there. I imgine some days are busier then others, especially since we're in Open Beta and the demand is increasing.