@ChristineSharpe
There is a bug in Canvas related to submission records not being created. I recently wrote about in Speedgrader not showing all students for certain assignments, but it affected over 680 assignments for our courses and other people are seeing it. One of the symptoms is that students cannot submit assignments and they do not show up on the To Do list for them. It could apply to the student not seeing it at all in a module. For more details, read that post.
The quickest way to check to see if it is an issue is to load SpeedGrader for an assignment with this issue. Check to see if all of the student names are there. If some of the student names are missing (and you're not filtering them), then it is likely this issue.
It seems to predominately affect assignments without due dates. Of those 680 assignments, less than 10 had due dates. This is because editing the due date fixes the issue of the missing submission record. If no one has submitted, you can also unpublish and then republish the assignment. That worked for about 675 of the 680 assignments we had. For the others, I had to edit the due date, save it, reset the due date to what it was, and save again.
That may not be your issue exactly, but it's worth a quick check. If not, you've only wasted a minute of time. This could even be something the instructor does in case there is a delay between when they notice and you hear about it.
As for the delay, it may depend on the type of assignment. There may be other things going on in the background that are causing those missing submission records to be created. If the instructor went in and edited the assignment, it sends the due date back to Canvas. That may be enough to trigger it, but I'm not sure.
