screen lock intermittent in 16.04.1 KDE

I have my KDE screen locker configured to "Lock screen automatically after" 1 minute. Sometimes it does, sometimes it doesn't. When it doesn't, I can lock manually with Ctrl-Alt-L. I upgraded from 14.04 to 16.04.1 ~4 days ago, and it has stopped locking 3 or 4 times. It was working fine in 14.04. It's particularly annoying for me because I use my screen saver status and my workspace name to log my hours to various charge numbers for work. If the screen saver isn't on, I'm logging hours.

I wasn't sure what component to file a bug against. I assume libkscreenlocker5 (there is no kscreenlocker package AFAICT).

apt-cache policy libkscreenlocker5
libkscreenlocker5:
  Installed: 5.5.5-0ubuntu1
  Candidate: 5.5.5-0ubuntu1
  Version table:
 *** 5.5.5-0ubuntu1 500
        500 http://us.archive.ubuntu.com/ubuntu xenial/universe amd64 Packages
        100 /var/lib/dpkg/status

Other info:
I've tried 5 minute locking intervals and changing the interval during the KDE session where locking isn't working... no change.

I've looked through all of my workspaces to see if there might be some dialogue window or panel widget expecting user interaction that might be disabling the timeout mechanism or something like that. I didn't find anything of that sort, but I just closed all windows except the firefox window in which I'm typing this question (emacs, thunderbird, octave, gramps, google-chrome, konsole, + ???) and the screen saver worked after that. I'm not sure if that was coincidence or not. I re-opened an instance of every app that I remember having closed and the screen saver worked again.

Does anybody have an idea what the problem with the screen locker might be or how to provide debug info for such a problem?