My Apple watch no longer unlocks my iMac. How to I fix it?

Great thread, and relevant to before Big Sur.

For me Auto Unlock flashes, as though it will work, then fails. Worked before Big Sur. Problem started with 11.3.1 upgrade. Watch, iPhone, MacBook updated to latest. Using deselecting, restarting MacBook, reselecting (as described in HT206995) allows Auto Unlock to work for a day, then fails again. Other ideas?

Check out this thread on fixing Apple Watch unlocking Mac

https://discussions.apple.com/thread/251803595?answerId=253458029022#253458029022