Unmatched Credit Notes.

Hello, I am fairly new to Sage, having previously only used it for data inputting. I am now trying reconciliation and all sorts of wonderful things. There are several accounts were credits have been raised, but not put against the invoice in question, even when for the full amount. This has resulted in invoice and credit for the same amount showing up as outstanding on reports. How do I post one against the other in order to stop the showing as outstanding. Many thanks.