A pre-payment will not purge if one of the following is true:

• The transaction date is greater than the purge-through date
• The transaction relates 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 a prepayment that has not been fully applied; that is, the remaining amount is not equal to zero
• The transaction is a prepayment that applies to a receipt 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?