Assignment not showing participants in one course only

Hi,

This is very strange, the assignment shows only one participants, then any additional submission after that from other students doesn't appear in the participants list ( see the image below)

Users are keeping submitting, but submissions are counted in the submitted count and those needs grading, but you can't view or grade them

Yet they are still available for download.

This is only happening in one course in my moodle 2.9.

Any assignment created under this course will behave the same, while any assignment under any other course work perfectly ??

I will be grateful if anyone has a clue