Well, there can be a couple of reasons why you can't connect to the master server.
First thing you should check is the version of your SRB2. Open the console (press the ~ key on your keyboard which is right next to the 1 on most keyboards) and type Version. If it is different then 2.0.4, then use the srb2 updater to update your srb2 to the proper version.
The second thing is where you live, and the time you want to play. There are certain times where more people host SRB2 on the master server. If you don't live in the U.S you might need to pay attention to when people nearer to you host.
Those are the only things I can think of off the top of my head.