Online Text Assignment ---Strange Problem

Hello everyone !

I am administering Moodle 1.9.7, but now facing a very strange problem:

Once a teacher set up an online Text assignment, all the registered students shows up, But as soon as students submit assignment, they disappear, therefore the tutor is unable to see their work . However, student can see their own work when they are logged in. As tutor can't see their wok so is  not able to  grade the assignment, same is true for the file submission.Me as administrator also can not see student's work.

Any solution please....

Thanks