The Visibility setting of an issue can prevent you from viewing it, so this is the first thing you should verify.
For example, if the author of an issue selects Access restricted to author, assignee, team and notify (like in the screenshot), the issue will be visible only to the author, the assignee, the team(s) in the Team involved field and users in the Notify field.
You can double check with that person to see if this field was properly set up upon creation of the issue.
If you're an Admin, you can make sure that most issues remain visible by adding yourself to all teams in the “Teams and Users” page. Once this is done, you’ll be able to search for and access all issues for which teams have been added.