I have multiple modules set up with links to videos followed by a quiz at the end.  However, several individuals (but not everyone) are not able to see the activity with an error message that comes up saying "Sorry, this activity is currently hidden." Currently, of about 300 individuals who have completed the module, only about 10 are not able to view it.  We assigned both modules as certifications, but with one of them having two main audience groups to encompass everyone, the other assigned to specific staff in specific departments.

Everything I can find in the settings for the Quiz, Course, and certification is showing that it's visible.  What am I missing?