Does your course have multiple sections in it? If so, it might be that in Speedgrader you've selected to just view one of the sections and thus only see those students. Click the triangle where you select students in Speedgrader and see if towards the top there is an option to select different sections or groups. If they don't see the Assignments it also could be that in the Assignment Settings in the Assign To area, you've selected a section rather than Everyone. Doing so would make the Assignment only visible to people in that section.
Rick
