Screen Saver Lock Button Does Nothing

Symptom:

Your desktop panel or menu has a button or entry for locking the screen. Press it. Nothing happens, screen does not lock.

What's happening:

A lot of things might cause this:

• xscreensaver or other screensaver is not installed. (Install it.)
• In the XFCE autostart menu, the screensaver is not set to be started on session initiation. (Should be on the list; click its checkbox.)
• The session's LockCommand has gotten deconfigured. (This was my case.)

Try in your terminal: xscreensaver-command --lock
This should reveal useful information. In my case the screen locked as it should, letting out the first two possibilities. This is on OpenSuSE Tumbleweed; other distros tweak the names differently, e.g. Manjaro calls it "xfce4-screensaver-command".

How to fix:

xfconf-query --create -c xfce4-session -p /general/LockCommand -t string -s "xscreensaver-command --lock"

Now, when you click the lock button the screen locks.