Change your grading methodology to max value if you'd like student mastery levels to be based on their highest score achieved while attempting to demonstrate mastery. Alternatively, choose to assign a greater weight to assessment types such as quizzes and exams using weighted average grading.
To access these features, go into "Class Settings" then click on "Gradebook." You can select which grading option you would like from there. If you are using weighted average, boxes should appear underneath each assignment type- assign what percentage you want for each assignment type. All of your percentages must sum up to 100%.