Excel add-in V9: Not able to drilldown

We recently upgraded from V7.4 to V9.1 and we’re seeing an issue with the Excel Add-In. A small group of users (about five) are unable to use the Drilldown feature (the option is completely greyed out), while the rest of our users can access it without any issues. All users have been upgraded to the 9.1 Excel Add-In and every other function appears to be working correctly.

Has anyone else run into this? Any ideas on how to fix it?