Student Assessments keep disappearing

We are having issues when using the assessment tools (multiple choice, single choice, and answer field). If you look at course OSSLT Preparation, you can see that all the answers students submitted have disappeared. This has happened several times in the past. They disappear in all the courses. I have singled this course out because these tools are used a lot in this course so it is more evident. What could cause this? HOw can it be prevented? Thanks for your help.