Grade average for one student is incorrect but the rest of the class average is correct in my grade book. What is wrong?
Is there a small dark triangle in the course grade column for this student? If so, you have manually overidden the grade and the gradebook is not calculating it.
Did you inadvertantly mark any assignment grades as exempt? If so, the scores will appear purple and italisized.
Without much more information from your end, this is not possible to troubleshoot.