LEFT JOIN Doesn't not give me the result i whant on a simple SQL request

Hello there,

I drive Crazy on a simple SQL statement.
I just try to get ALL accountnumber with the sum of Transaction line amount for a department, even if Amount is null.

I did that:
SELECT
    ACCOUNTS.ACCOUNTNUMBER, SUM(TRANSACTION_LINES.AMOUNT)
FROM CECI.Administrator.ACCOUNTS ACCOUNTS
    LEFT OUTER JOIN CECI.Administrator.TRANSACTION_LINES TRANSACTION_LINES
     ON   ACCOUNTS.ACCOUNT_ID = TRANSACTION_LINES.ACCOUNT_ID
WHERE TRANSACTION_LINES.DEPARTMENT_ID = 28
Group By ACCOUNTS.ACCOUNTNUMBER
order by ACCOUNTS.ACCOUNTNUMBER

The result does not populate all Account Number if there is no Transaction lines.
Someone can help me on this one ?