I am having the same issue. The watch was unlocking my MBP perfectly then just stopped. When I wake the MBP up from sleep the MBP says unlocking with Apple Watch. Then in the unlocking panel it has a spinning wheel. Then I need to add my password fro the MBP to unlock. Tried rebooting router, restarting the MBP, iPhone and AW all without success.
Then it worked again once and stopped unlocking the MBP.