A 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 receipt that cannot be purged
• The transaction is related to a job that cannot be purged
• The transaction has been paid by a credit memo that cannot be purged
• The transaction is a credit memo paying an invoice 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 sales order that cannot be purged
• The transaction is related to a ticket (Sage 50 Complete Accounting and higher only) that has not been purged
• The transaction is voided and either the original sale or the related void transaction cannot be purged. In this case, both transactions are left un-purged
• The transaction was reconciled after the purge transaction

[BCB:161:Chat 50 US:ECB] 

Why do certain transactions or items not purge?