In the section "Invoicing" of a relation file you will see a few boxes that indicate, among other things, how many registrations are still to be invoiced.
If you click on "Registrations to bill" you navigate to the overview page of the registrations. You may get the message "No elements in this list".
On the overview page of the registrations you have different filter options at the top, for example user based or period based.
By default, the filter is set on the user who is logged in when you enter this screen. If you see this message, the registrations may have been made by another user.
The same also applies to the period. It is set to "This month" by default. If the registrations are made outside this period, they will not be visible.
In the next AdminPulse release, the default filter setting will be "All Users" and you will first see all registrations when you enter this screen, and then fine-tune yourself.
In the meantime, you can of course change the filter yourself.