Open invoices but no balance

The customer's balance should be the same as the sum of transactions - for all time - that use an Accounts Receivable account.

Sometimes the balance can get corrupted when QuickBooks crashes when recording transactions, where the transaction updates but the name balance does not, or the other way around.

However, it's more likely that the customer has more transactions but they're not in view on the customer center, and instead are filtered out. 

Try setting the filters above the table of transactions to show All Transactions for All dates. After you do, then do more transactions appear? You might, for example, see some Payments with open balances as well.