Mac Pro 5,1 won't sleep anymore

So my 5,1, which has been great for the past few years, has stopped going to sleep lately. When I hit sleep from the Apple menu, the monitor goes into Energy Saving mode, and....nothing else happens. So far, I've tried an SMC reset, unplugged everything except mouse, monitor and keyboard. I also checked Activity Monitor under Energy. And it said nothing was preventing sleep. And the Energy Saver preference has "Wake for network access" unchecked and "Put hard disks to sleep when possible" is checked.

I am running Mojave, latest version 10.14.6. The only software I've updated lately is a Firefox update to 76.0.1.

Any ideas??