Hi Erica,
I've never been a great fan of setting a "grade" for the Questionnaire activity. I can be confusing and lead some users towards thinking that a student can have a different grade according to the way he answers the Questionnaire questions, which is definitely not what a survey-like activity such as Questionnaire is for.
The only use of that "grade" is in the way of encouraging students to answer a questionnaire. As you have found out, that "grade" is given once for all, the very first time a questionnaire is submitted.
Joseph

