Comment 9 for bug 473705

Some applications disable the screensaver (and power management) when they're running. qemu is one such application.
The times I've seen this were when I was running qemu.

you can stop qemu (and actually, all SDL applications) from disabling the screensaver by putting SDL_VIDEO_ALLOW_SCREENSAVER=1 in your environment.

One place to do something like this is in ~/.bash_profile with:
export SDL_VIDEO_ALLOW_SCREENSAVER=1

This may not be your problem.