When a customer makes a partial payment, QuickBooks recognizes the payment as paid in full and when you send the invoice again, it doesn’t show a balance.

The work-around to this is; un-apply the payment in QB (DO NOT delete the payment as it’s probably been deposited but un-apply it form the invoice. THEN resend to customer so they can pay the remaining balance.

Don't forget to apply the initial payment to the invoice in QuickBooks after you've sent the invoice again