Why won't the monitor go to sleep?

"A possible issue is the sensitivity of some mice, especially gamer ones.
Mine, for example, often keeps my laptop from turning off the monitor.

Check if the system is actually detecting that you are idle, for example, launch some messenger clients, and see if they detect that there is no movement. If yes, then it's probably an issue of settings, like the other answer. If not, if they see you as ""active"", then it means movement is detected from mouse, or maybe from screen, since you use Tablet edition."