Errors

Some students can't view their grades on blackboard.

For example, suppose you had an online assignment, but one student turned it in in hardcopy, and you read it then entered the grade in the grade center spreadsheet view.  In that case, there's no attempt and no attempt grade.  The grade you entered is treated as an "override" for the assignment (after all, Blackboard has no way of knowing which attempt to associate it with...).