Sounds like you are doing everything correct.
I would try -->
Gui ->
1---Try different screen saver in case there is a issue with that and not the application
Terminal -->
2---gnome-screensaver to launch and see if you get errors.
3---
a--apt-get purge gnome-screensaver ( uninstalll & remove any configuration files)
b---apt-get install gnome-screensaver ( install hopefully fixing any bugs)
Hope this helps.