I created an assignment using Flipgrid, but can only see one section of students in the Speedgrader.

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My course is set up so there are three sections that I cross-listed into one course.  I used Flipgrid as an external tool to create an assignment for students.  I created three separate assignments, one for each section of students in the course.  When I try to use speedgrader to grade the videos, I can only see students listed for one of the assignments.  In the other two assignments, there are no students listed at all.  When I look at the grid, I can see that students in those sections have posted videos on the other two assignments.  I don't think any of my students are accessing this outside of the Canvas assignment, which is one of the things that Flipgrid lists as a potential problem with speedgrader in their directions.  Anyone have any idea why I can't see students in the other two assignments?