by M Horn.
Not to fear, this problem was not Moodle-related, but a bug on Turnitin's side:
You are receiving this email because you are the administrator of a Turnitin account that was recently affected by a bug that existed between January 22 and 29th. During this period, submissions that were made to Turnitin assignments via a “basic” or “framed-in” integration did not update in the grade book or grade center within a Learning Management System (LMS) due to a failed web services call. In essence, a student would submit her paper successfully, but the instructor would not see that a paper has been submitted by this student. Users accessing directly through Turnitin.com (or Submit.AC.UK) were unaffected.
So all's well that ends well!
M Horn