The statistics reporting tool does not capture student data for assessments if the students accessed the assessment through the Lessons tool.
Make sure the statistics preferences is checking for Test & Quizzes activity
Setup two assessments in the Test & Quizzes tool
Link the second assessment through Lessons by the 'add quiz' link
As a student, access and submit the first quiz from within the Test & Quizzes tool.
As an instructor, pull a report for student activity and you will see the students 'take' and 'submit' event captured.
As a student, access the Lessons tool, and take the linked assessment.
As an instructor, pull a report for student activity. You will see the student accessed the Lessons page, but there is no events shown for the student taking/submitting the assessment.