Mabel:

It would help to know which version of AR you are using.

Something that might be a problem include;

Using 'Calculate as of' date prior to the date of this credit note. This is a cut off date.

Using some range selection that cuts off this customer or transaction.

What you can try is to use the 'Aged as of' optioon as this includes all outstanding transactions and making sure all of the range selection filters are set from <blank> to ZZZZZZ (fill all possible characters as z's).