Time in Session Data not populating in attendee report?

In my attendee report for my latest webinar only ~50% of the data lines have the time joined, time left, and time in session.  Why is only half of the data populating? We required registration ahead of the event and all of the other attendee data populated in the report. I also watched the participant count during the webinar and it aligns with the # of attendees reported. I just don't understand why the time fields are showing "--" instead of data?