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.
Please check your sync connection if you are using QuickBooks Online, or the Method Sync Engine if you are using QuickBooks Desktop.
If there are no conflicts, you can manually start the syncing process.