Why won't the screen saver start?

When I walk away from my desk, I like to start the screen saver. I do this to "lock" my laptop, requiring a password to be entered when coming out of the screensaver. I use the upper-right hot corner to invoke the screensave. Very infrequently it doesn't work, I just can't start the screensaver. It appears that a reboot is the solution to reset everything. Any thoughts?
 
indifferent I’m unconcerned
Inappropriate?
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