If some of your students report being unable to see their grades while others report otherwise, then your students may not realize where to find the link that takes them to the gradebook. If possible, give your class a brief orientation on how you have set up the gradebook and how they can find it from your course's main page in Moodle.
If students are reporting that they cannot see grades for your course at all, then your course settings may be set to prevent students from viewing grades.
If students report they can see some grades for your course, but not a particular graded activity, then check to see if the activity is hidden from students. Hiding a graded activity on your course page also hides its grade from students.
On your course page, click Turn editing
Identify the line item that is hidden and select the edit drop down menu on the right side of the screen
Click Show and the item will now appear in the gradebook.
Has a student's grade for an assignment disappeared from your Moodle gradebook? Try either of the options below to see all the grades you have already entered. Click on this LINK to locate instructions on making Moodle Grades visible that may have been previously hidden.