Even when an assignment if graded fully automatically, for example when all questions are multiple choice, you still need to approve the score and, hence, the grade. Only after approval the grade is shown to the student. Also a notification is sent to the student.
When there are still grades to approve, you will see this with a red indicator in the Grading item in the left panel of the course. In this case, click on Grading, pick one of the assignments where there are more submissions than graded submissions, and press the button Grade submissions.
You are now shown the report for the first submission. At the top right you see the computed grade and a button to approve it. If you are happy with the grade, press the Approve button. The scores and grade are now fixed and can no longer be changed. They are now also visible to the student. If you don't agree, go to the different questions and adapt the score.
For an approved assignment you can press the Unapprove button to change the scores again. But you are strongly discouraged to do this, because the student already received a notification about his/her grade.