Check Grade item for activity is visible (eye should be open) - If the eye has a line through it, the grade item for the activity is not visible to the students. Click on the eye to make that grade item visible and check again.
Check Eye is open next to each student's grade - If the eye is closed, the individual grade for that student is hidden. Click on the eye to make that grade visible, or hide and show the whole grade item and check again.