Sales Register does not agree with client ledger

I have a credit for a customer in the sales register but on their customer ledger the balance is zero.  If I delete the credit note then the ledger changes.  If I raise an invoice to apply the credit then I have a balance on my ledger but nothing in the sales register.  I need to get the sales register to have a zero balance for this customer as well as the ledger. The ledger is correct so I do not know how to rectify this.