Mac often wouldn't sleep nor show screensaver If I manually &Lt;Menu &rarr; Sleep&Gt; or &Lt;Screensaver settings &rarr; Preview&Gt; it will work, but it wouldn't do sleep/screensaver with the set hotcorner nor automatically.

Every now and then I'll have this issue. How do I fix the problem or investigate what has went wrong?



(Its usual settings: https://i.stack.imgur.com/RcJH5.jpg)

(Refreshing the settings (eg by toggling &Lt;Enable power nap while plugged into a power adapter&Gt;) would temporary fix the problem&mdash;until it comes again.)
Launch Terminal 

pmset -g assertions


that will give you a clue as to what is stopping sleep