the problem is that in the form an ungraded quiz question is displayed as having a grade of 0. And if you submit a grade of 0 it is treated as if the grade has not been changed and the new grade is not stored.
I think this needs to be changed so that nothing (an empty string), or maybe '--' appears in the grade field when the question is ungraded. And when a grade of 0 is submitted this is detected as a change and stored in the db.
May try to code a fix for this if I have time. Assigning it to Tim for now.