screen saver not starting

sometimes something is running that is "Preventing Sleep' if no sleep, no screensaver.

this discussion answer form another site may be helpful:

1. The next time your Mac isn’t going to sleep, open Activity Monitor
2. Click on the Energy tab
3. Now you’ll see a list of applications with a number of columns, one of which should be headed “Preventing Sleep”
4. Now check to see if any applications have a “Yes” listed in this column

from:

https://apple.stackexchange.com/questions/364622/how-can-i-find-what-program-is-preventing-my-mac-from-going-to-sleep

Readers are eager to discuss this further., just post back.