On the View/Grade all submissions page of assignments. Several clients have reported that specific teachers don't see all their students on that page. What I've seen is that they haven't selected "All" for First name and Last name near the top of the page. Clicking "All" for both (depending on other filters at the bottom), usually fixes the problem.

It seems that if a teacher has filtered on a students' name in one course, the filter is saved as a preference and it will remain that way until the teacher selects to view all. The preference is saved even after the teacher has logged off the site.