Unable to tag user in comments

I am sometimes unable to tag 2 users in comments. When I type the name in the comment box, it doesn’t come up as a link. How can I fix this?

It works in some files but not others. One example: I created a new Figjam, set Permissions to “anyone at my org can Edit”, then tagged 1 team member without issues, but another would not show up in the list even though this user is taggable in other files.