@Chris_Hofer, thanks for tagging me.
@rangle,
Is the instructor information on the Data sheet? If so, it's a problem with the pivot table.
You may have a filter to select only students and not teachers. Edit the pivot table and check the filters. If the pivot table controls on the right side are closed, you'll need to go to the bottom of the pivot table and click the Edit button to get them back.
A second potential issue is related to pivot tables. Pivot tables don't update if you have filters applied. Edit the Pivot Table configuration, remove the filter, and set it back if necessary.
I had a table like this on my copy of the spreadsheet where I checked your issue. It downloaded all of the information correctly, including teachers, but it was an old course. I had a filter set to active students and the enrollments were all completed, so nothing was showing. I changed the filter and then information came back.
If the instructor information is not contained on the Data page, that's a different issue; one I'm not able to duplicate.
My guesses in that case are that the instructor didn't make any posts or that the Discussion Redesign has affected things somehow. I would start with the filters in the pivot table.