Why isn't my student showing up on my dashboard?

The most likely reason for this is that the student already had a Grok account, but it was not associated with your school.

For privacy and security reasons, since your school did not create these accounts, you will not be able to see them until the student agrees that they can be associated with your school. When these students next log in, they will be prompted to allow or deny your school access their account. Once they have approved this request, their account will appear on your dashboard.

If the student has accidentally selected to not be associated with the school, they can add the school to their account themselves via  Account > Institutions.

There is a second possible reason, which is that the student account has expired. You can read more about account expiry here.