@scourj
Do the students who couldn't take the quiz show up in SpeedGrader for that quiz? There was a known issue with imported quizzes with no due dates not creating submission records and students not being able to take them. Canvas fixed the issue with the import that didn't create the record on January 15, but they didn't go through and fix the missing submission records. I had to fix about 680 assignments (not all quizzes) in 80 classes for this term.
I'm not sure what it looks like from a student perspective, but I was alerted because the teacher only had a couple names in SpeedGrader when they should have had a lot more.