Why is my Superbill showing that a session hasn't been paid when I have already entered that payment in my client's profile?
Superbills will only show that a session has been paid if the invoice attached to that session is marked as Paid. Keep in mind that Invoices aren't able to record partial payments. That means that you have to make sure to enter the full amount of the invoice in order for it to be marked as Paid.
In this case, start by deleting the Superbill. Then, go into this client's Billing page and make sure there is an Invoice attached to that session. Make sure you've applied the payment to this invoice by opening it and clicking the Pay button on the top right corner.
Once the invoice is marked as Paid, you can re-create the superbill and it will show that this session has been paid.