Looking at this again, I think it is that you do not have access somehow to this specific group of students in this course. Obviously the students are able to submit so they have the correct access. It is more that you do not have access to see the submissions.
By chance, are you in the same group as Student A and not Student B students?
When you click on View All Submissions - is there a group dropdown list at the top? Is there an option to View All Submissions from the grading screen?