When recording a payment from a customer, the customer's balance may not immediately update after the payment has been entered.
This is because the balance of a customer is synced from QuickBooks.
If a payment is successful, but the balance does not update, it's usually due to the sync from QuickBooks.
There are a couple of reasons why the sync may fail:
The sync is disconnected.
If there are no conflicts, you can manually start the syncing process.