I have one user in Jira that I can no longer assign issues too or find using '@'. I used to be able to assign things to this user. Some others can see the persons name in 'Assignee' etc. but some others can not and the user can assign theirself to issues.
I have tried rebooting, using a different browser (in case it was a caching issue), logging out and back into JIRA?
Any suggestions?