Only create bank transactions from the statement (do not create them using the regular ways). When you have created a bank transactions outside of the tool, attach them to the statement using the 'Show Closed Transactions' checkbox on the Select Transactions popup.
Check the start balance. Was the bank balance in NetSuite correct before entering this statement? This will help where to look: either in this statement or before. To find out, look at the Account Register for the bank account (Lists > Accounting > Accounts, click on the bank account).
After making changes to earlier bank statements, edit and save all newer statements, because the Bank Balance NetSuite on the statement is only updated on save.
Search for transactions outside of the bank statement: go to Transactions > Management > Search Transactions. Enter the following criteria and click Submit of Preview. The results will show transactions that are not linked to Criteria: