Apple Watch unlock Mac problem

I love unlocking my two Macbook Airs (older models, around 2015) and my iMac 27" (2020) with my Apple Watch (the last gen, 5th I think?). On my new iMac 21.5" (2020) though, it rarely works even though it is setup the same. I was wondering if it's because this computer my spouse also uses on a separate user account, because otherwise I don't understand why it doesn't work. It seems that it should work even if other users are logged in since I would only use the Watch to log in to my user account but not sure how it works in that situation.