When you are running the AP report (I am guessing you're using the Historical Aged Trial Balance?) what setting are you using for "Select Transactions for Report Using"?
If you're using GL Posting Date, what dates are in the following fields in the payables apply table (if the credit memo is fully applied, it should be PM30300):
GLPOSTDT
ApplyFromGLPostDate
ApplyToGLPostDate
If you're using Document Date, what dates are in the following fields in the payables apply table (if the credit memo is fully applied, it should be PM30300):
DATE1
APFRDCDT
APTODCDT
