The most common explanation for the difference is that there are unreconciled transactions. For an explanation on how to reconcile your transactions, please see the article here.
If you have no transactions to review, one of the main reasons why your bank balance differs to your bank statements may be due to the transactions being pending. Some banks will create the pending state and remove the balance from your account, so that you are unable to spend it. This would only push through once it has been confirmed.
The first thing to check is that the missing transaction is confirmed with your bank. Once it has been confirmed, it should show within your transactions.