I have been able to unlock my iMac with my Series 3 Apple Watch reliably until the last couple of months. But then it stopped working. I discovered (though this forum, thank you!) that if I signed out of Apple ID on my iMac and then signed back in, that would solve the problem.
The trouble is that I have had to do this about five times in the last month. I've learned that I don't need to save the data in iCloud to my iMac – that can take a while.
But even so, it does take some time to sign out and then back in.
I had this problem before I installed Catalina and continue to have it now.
Any suggestions on a permanent fix or is this one of those MacOs problems that Apple needs to fix?