I can think of several possible things that could have occurred:
- The student was logged in as a guest (or as another student), as Gavin suggested above.
- The student did the work after the due date and so the answers should not be recorded (they would have "Check Answer" rather than "Submit Answer" buttons, but the student might not have noticed it).
- The student hit RETURN rather than the "Submit Answers" button, and so had their answers previewed but not graded.
- The student's answers were recorded, but when the student logged in again, the browser showed a cached copy of the problem set page that was from before the student completed the problems (I have seen this happen).
- The student actually didn't do the problems (in time) and is trying to get credit for it after the fact by suggesting the system is at fault.
Again, the log out process is completely unrelated to the recording of answers, and I don't see how that could have any effect on what answers where actually saved for the student. If there were such an issue, you would be hearing from a lot more students, I'm sure.