A vendor credit memo will not purge if one of the following is true:

• The transaction date is greater than the purge-through date
• The transaction is not fully paid; that is, there is still a net amount due, either negative or positive
• The transaction has been paid by a payment that cannot be purged
• The transaction has been paid by a vendor credit memo that cannot be purged
• The transaction is a vendor credit memo that pays a purchase that cannot be purged
• The transaction is related to a job that cannot be purged
• The transaction uses a general ledger account that requires reconciliation and the transaction has not been reconciled
• The transaction is related to a purchase order that cannot be purged
• The transaction was reconciled after the purge-through date

[BCB:161:Chat 50 US:ECB] 

Why do certain transactions or items not purge?