Why Does a Student Grade Not Change When I Enter a Zero for an Assignment?

[TimDaniel]

There is a circumstance where this can occur.  If there is an assignment category that has assignments in it, but the student has not yet achieved any marks for any assignments in that category, their mark will not change when a grade of zero is entered for a subsequent assignment in that category.
 
For example, suppose your class has two categories, Assignments and Portfolio, each worth 50%.  While there are no assignments in the Portfolio category, the mark will be calculated out of 100% for the Assignments category.  However, as soon as at least one assignment is entered in the Portfolio category, the category now is worth 50% of the total mark and the student's grade will include the marks in that category. 

In this situation, the student mark will change if a zero is entered in the Portfolio category.  However, PUTTING IN A ZERO FOR A SECOND ASSIGNMENT in that category will not change the mark - the student has no marks in the category, and an additional zero does not change that.

Note that if all the assignments in that category are excused (left blank, or "exc" is entered), the mark will once again be based on the other categories only.