Why isn’t my Apple Watch unlocking my MacBook?

For reference: Unlock your Mac with your Apple Watch - Apple Support

Assuming you did all that, the following is what worked for me after waiting a few days with no resolution (which had always worked in the past, until one day it didn't):

1. Turn off WiFi on Mac
2. Turn off "Watch Unlock" on Mac
3. Shut down Mac
4. Shut down Watch
5. Turn on Mac
6. Turn on Watch
7. Turn on WiFi on Mac
8. Turn on "Watch Unlock" on Mac

Done.

Just some random thoughts (using macOS 15.3.2 and watchOS 11.3.1 as points of reference)

1. Verify that your macOS and watchOS are on their most current versions of their operating systems.
2. Verify that your Mac's Setting->Touch ID & Password->Apple Watch (bottom of web page) toggle is turned on for your Apple watch that uses the same Apple Account.
3. Verify that Bluetooth feature is turned on with the Mac and the Apple Watch.
4. Verify that your Watch battery has plenty of charge (as well as your Mac if working off of the battery).
5. Verify that your Apple Watch is not in low power mode or low on power (I'm guessing a bit here).
6. Make sure your watch is snug enough ON your wrist. If you have more than one Apple Watch with both activated with this feature on, I assume that the one on your wrist should work.

I use the feature a lot but I can also empathize with the poster. Mine works most of the time, but I have not taken time to figure why it did not work, but it is rare for me. If I come across anything official, I will post it. I hope this is a useful set of first steps and not too basic. :>)