This is a question has been sent to support via email and been answered/resolved. I am posting it in the forum so hopefully it would benefit others.
The question/issue is described below:
have a student who is enrolled in the current year – I can log in as the student, use the course selection, admin can approve the course selection, etc. However, when I log in as admin to issue an invoice for that person, I can NOT find that person in the list.
The answer is:
In short, when any new students are added, you need to head to Other > Finance > Manage Invoicees (note the double e…as in people who receive invoices). This will create an invoicee record for any student who does not have one.
After getting the answer, I went to the paged as directed. Didn’t do anything on the page, and then go to issue an invoice, the missing student appears in the list.
It seems that by simply visiting “Manage Invoicees” page, solves the problem. It is possible the background process/queries running when opening that page is the solution. 
Cheers.