This usually happens when the date of the bank transaction is significantly different to the date of the transaction entered into Cashbook.
For example:
A cheque may be written on a certain date but this is not deposited into the creditors bank account until 10 days after the cheque was written and entered into Cashbook. Or,
Cashbook automatically generates a transaction to record the net amount of payment (or refund) when completing a BAS return. The payment or receipt may not be processed by the ATO for more than a week.
To fix these cases increase the No. of days to show matches in the Find Matches window
For a more detailed instruction click on the following link: Match
Article ID CBP1497