yes. there are many reasons for "dead" servers in the usgn list.
wrong firewall or router settings are only two of these reasons. another one is a crash of cs2d or the system, an internet disconnet while playing or an untidy shutdown of the cs2d server (with ctrl+alt+del for instance).

its absolutely common that only some of the servers in the list are reachable.

the usgn for cs2d is very simple due to the lack of a real server. I had to develop a system which works on ordinary webspace and which does not cost much cpu power. the result of this are these dead servers in the list.

just ignore them. they should not hinder the usgn.

and as marcio says: at the moment there are a lot of players active. when many different players start servers with wrong firewall and router settings there will be a lot of dead servers in the the list.