The 'Attempts Statistics' feature is displaying a message that 'No one has yet completed the assessment', even though student attempts show in the Grade Center. This happens when the 'Highest Grade' option is used for the 'Score attempts using' feature and the test allows only a single attempt.
Steps to Repeat:

1. Create a test
2. Set the Test Options to allow only one attempt
3. Set the Score Attempts using option in the column Information to "Highest Grade"
4. After students make attempts, run the Attempts Statistics from the Action Link and Contextual Menu for the test, in the Grade Center