The known issue where instructors were unable to email non-submitters has been fixed.
To email those students who haven’t submitted an assignment, visit the assignment inbox (via Control Panel -> Course Tools -> Turnitin Assignments).
If you think students may have been added or removed from the Blackboard course click Roster Sync [1] first – this will ensure the students in Turnitin match those enrolled in Blackboard.
Then, click Email non-submitters [2] where you can compose a message to be sent to students yet to submit any work.