Group by failing on Null values

I have a SELECT query that basically adds up my sales, removes credit
adjustments (eg. Returns) and gives the net figure.

This is the query:

SELECT (SELECT SUM(cli_tran_amount) FROM vetpmardet WHERE
cli_credit_adj_trans_no IN (SELECT sys_tran_number from vetpmardet WHERE
cli_tran_trans_date BETWEEN '2004-07-11' AND '2004-07-17' )) +
SUM(cli_tran_amount) AS amount FROM vetpmardet WHERE cli_tran_trans_date
BETWEEN '2004-07-11' AND '2004-07-17' AND sys_transaction_type LIKE 'C'

The subquery in there is necessary to link the credit back to the original
transaction. This query works well as far as I'm concerned.

Where I run into problems is when I try to break the report down by staff
codes. Unfortunately, my accounting s/w does not put a staff code with the
credit adjustment, but I do want to be able to see the net sales by staff
code. So I modify the query like this: (add one select column and a group by
clause)

SELECT dat_staff_code, (SELECT SUM(cli_tran_amount) FROM vetpmardet WHERE
cli_credit_adj_trans_no IN (SELECT sys_tran_number from vetpmardet WHERE
cli_tran_trans_date BETWEEN '2004-07-11' AND '2004-07-17' )) +
SUM(cli_tran_amount) AS amount FROM vetpmardet WHERE cli_tran_trans_date
BETWEEN '2004-07-11' AND '2004-07-17' AND sys_transaction_type LIKE 'C'
GROUP BY dat_staff_code

But this query will not produce a row for dat_staff_code when it's value is
null. It only produces grouped rows where dat_staff_code is not null, and
the sum of those rows does not equal the value of the single row returned in
the first query above. How do I get it to produce a row even when
dat_staff_code is null?

TIA

Caleb