[LightDM] Login screen is not always coming back

[MR ZenWiz]

I noticed a few days ago that my secondary login screen, usually
accessible through the screen saver pop-up, was not coming up.  All
that showed was the cursor in the upper left corner of the screen.

Yesterday, in an effort to see if a logout-login would clear this up,
I discovered that the main screen (console 7) logged out with the
usual text trails on the screen, but the login window never came back.
 I had to reboot, at which point all was working well again.

Is this a lightdm feature or failure, or something else?

I'm running Xubuntu 12.04.1, usually continuously logged in between
kernel updates.

Thanks.

MR