Invoice shows as paid but still appears in the Receivables report

Not sure if this query is in correct location. But here goes. Had a customer pay an invoice in June. It was entered against his account and June reconciled and matched my bank statement. It appears as paid on his card and appears as paid in the sales register. HOWEVER, it appears as outstanding when I bring up receivables. 

I only discovered this when I sent a statement with the outstanding amount only to have the customer give me details of his payment, which matched what I entered.  How many other customers are going to ring me to say they have already paid.

How can this possibly occur. And how do I fix it.