If your screensaver isn't working as it should be, make sure it is enabled. Find the screensaver settings under Settings > Personalization > Lock screen > Screen saver settings. If you don't currently have a screensaver selected, choose the one you like and set the amount of time before it activates.

What prevents screensaver from starting?

The screensaver not activating could be caused by three different issues: (1) a program is specifically preventing the screensaver, (2) the mouse is preventing the screensaver because it is “moving” due to a hair on the optical sensor (or dirty ball) or being perched on the edge of a desk/pad/etc, or (3) a program or ...

How do I force my screensaver to start?

Right-click on the desktop, choose Personalize, and then click on Screen Saver on the bottom right-hand side of the window. Now you'll want to configure your favorite screensaver.