This issue has been resolved, following a patch from Bb. Please contact firstname.lastname@example.org if you continue to have trouble with this feature.
Problem: The “Last Access” column entries for some students are appearing blank or inaccurate in the Grade Center, Performance Dashboard, and Retention Center however those students have recorded activity in the course.
Solution: We are working with Bb to find a solution to this problem as soon as possible, and have been told a patch is in development. In the meantime, instructors can check student activity by using the course reports feature. This can be done by going to Control Panel -> Evaluation -> Course Reports, clicking on “Overall Summary of User Activity”, and selecting all students and a date range. There is a section of that which that shows a student’s access per day.