When the option 'Require a minimum letter grade of' is selected on a Degree, the grade is determined based on the marking column selected to use in requirement calculations.  If this field is blank, or a different marking column is selected than the marking column in which  the student received credit, it will not calculate toward the requirement.

To verify or update the marking column, select Configuration, Business Rules, Degree Audits.  In the field Marking Column Grade to use in requirement calculation and display:, select the desired marking column.