Can't Add comments message for a student on an assignment

Hi, 

I have a teacher who has contacted me about an assignment.  The teacher has graded the assignment but wants them to be able to add more files - It appears that the files have been deleted somehow.  But he has already graded it.  Now it's my understanding that once an assignment has been graded then a student can't add more files.  He should see his grade.  The problem i'm having is that when a student goes onto the assignment it sends him to a page which simply says you can't add comments.  When I follow the link with more infomation is says that this is the error

error/moodle/nopermissiontocomment

Can someone shed any light on what is happening?

Thanks

Kieran