When you go into Speddgrader do the student who were unable to submit/take the assignment show in the student list?
At my institution, we've run into a small cache/bug issue where students are missing from the Speedgrader student list. When they are missing from this list they are unable to take/submit and even access an assignment. To them, there is no link to click on and no notice appeared on their To-Do List.
We are able to see the students who were impacted by downloading the gradebook (full export), you will see “N/A” as the grade for a student who encountered the issue. To resolve the issue for the students of a "closed" assignment, you will need to either reopen/extend the due dates for the assignments or create a new “Assign To” group for the students who were impacted.
For future assignments that are showing an “N/A” as the grade for a students, we were able to just edit the release date, save, and then revert the date back (and save), and the “N/As” all cleared out.