One answer seems to be missing from the suggested problems.
This is that the user needing the mention does not have the permission to see the project.
In Project > Admin > Permissions, ensure that the browse Project permission contains something that the missing user mention is within.
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
We're bringing product updates and pro tips on teamwork to ten cities around the world.Save your spot