Unlocking new Mac not working with Apple Watch

I've grown accustomed to the convenience of being able to unlock my MacBook Pro with my Apple Watch, but I just got a new Mac Mini, and no matter what I do, it is not unlocking it for me. I have checked "Use your Apple Watch to unlock apps and your Mac" in System Preferences → Security & Privacy → General. And I am able to use my watch to unlock (e.g.) settings in the Privacy tab of that same System Preferences pane, like Accessibility settings. I just *can't* unlock the computer itself. What could be wrong? Why would this work on one computer and fail on another (newer) one?