Another trick is to open Activity Monitor, switch to the Energy tab, and sort by the "Preventing sleep" column. Some apps prevent macOS from sleeping.

In my case, I've discovered that Devonthink (document/notes management app) is responsible. I've been meaning to file a bug report about it.

I'm surprised that Apple's power management doesn't have an alert for this. Surely an app that causes my Mac to become glowing hot while sitting in my backpack, not to mention slowly running out of battery, is a pretty important thing to intercept. Meanwhile, I keep being asked if Chrome should be allowed to find devices on my network, which doesn't seem nearly as important.