Hi there!
The most common cause is that you can't join a full server via the UI and trying will tell you it's full.
Make sure you use the `connect` command in the developer console - this will skip the initial UI check and let you in if there are still reserved slots available.
Note we do cap the slots at 26 players, so if there are already that number of players in the server, the server is truly "full" until someone disconnects.
Please also note that it can take a few minutes for your account to be registered with our servers - usually 15 minutes plus a map change as this is the cycle on which the servers get updated lists of supporters.
~Vintage