It works all the time on my late 2015 Macbook. It never works at all on my late 2014 5K iMac. I’ve given up trying to figure out why…
On my macbook pro 2015 it works almost always (ca. 99% of the time)
Two Macs:
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Macbook Pro 13inch Late 2016
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iMac 27 inch 2017
I have no issues at all to unlock both Macs witch my Apple Watch.
Hopefully you purchased Apple Care+ with your iMac
$169 for 3 full years of Apple support…it’s the best bargain Apple offers…
They can assist you 24/7