Hi Michelle,
Previous assignment submissions are not stored and are deleted to enable the user to make a new submission. We are updating our documentation in order to clarify this behaviour. I'll drop a note to our Technical Writers & take it up with our Product team - probably there should be a warning in the UI so that users are clear on what they need to do - in some cases for example an appeal could be lodged on the grading and obviously it would be crucial to have a copy of the original assignment.
cheers,
George.