are you using nvidia as the X driver? If yes, check if this is a duplicate:
https://launchpad.net/distros/ubuntu/+source/gnome- screensaver/+bug/45018 -- No screensaver if xinerama is enabled https://launchpad.net/bugs/43619 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
