Find my Mac is an interesting iCloud feature that allows us to protect the Mac a bit against theft, but many people are reporting that it does not work properly and they fail to make the Mac discoverable.
There are several solutions for this on the Internet, but the one that has worked for me is to activate the location services, which - without doing anything - had been deactivated for who knows why.
To activate them you have to go to System Preferences> Security and Privacy> Privacy and "Activate location services".