After typing in the IP address of the VNC server, did you enter the colon and the VNC session number? For example, I know that TightVNC doesn't use the active X Window session (:0) by default, so it starts another X Window session (usually :1). Thus, you would have to type the following in CotVNC:
Also, are you sure you have instances of TightVNC running on your GNU/Linux server?