Provide more information, if you wan't to get help.

I assume you try to connect via internet.
This is a well known issue caused by the steam implementation, that doesn't allow to pass NAT of your router. You need to forward the necessary ports in the router configuration.

http://feedback.arma3.com/view.php?id=19302
http://www.reddit.com/r/arma/comments/28hw92/cant_play_arma_3_multiplayer_with_friends_now/
and many other ...