Problem|"Failed To Connect"

Hey everybody,
I build a game with multiplayer.
I give the game to my friend and play together.
I using MasterServer.
I find the server of my friend on the Server List, I click on “Connect” and waiting but…
It’s writing “Error: Failed To connect”…
Why!?
In tried to play with anothr friend and… it works…
And with another friend it’s don’t work too…
WHT!?
Please help me!

UP.

checked firewalls on both ends?

The firewall need to be on or off?
Because We playing when the firewall is on…
Still, What I need to do.

Server : Open correct ports ( YOU NEED TO KNOW EXACTLY WHAT PORT ), forward correct ports, disable modems firewall DISABLE DHCP and make your ip STATIC.

If you cant do those , dont do multiplayer game - there is enough tutorials on google how to do all of that, just search!

How I disable “DHCP”?
And, How I make my IP “STATIC”?

http://www.google.fi/search?hl=fi&source=hp&biw=1646&bih=926&q=How+I+disable+%22DHCP%22%3F&aq=f&aqi=&aql=&oq=
http://netsecurity.about.com/od/quicktip1/qt/qtwifistaticip.htm

it’s link that explane about both the things right?
DHCP and STATIC?
Edit:
Oh and…
If I do both things it’s will works?

you must understand how network protocol works, then you can solve your problem