Need Help On SQL Query On Reporting Services

Although not totally related to this RS cat but since I'm using the query in Reporting Services perhaps I can get a work around from someone. I've created a Rolling 2 year 12 month comparison report that compare sales figures from current and previous years. The query works fine however the query fails to return 0's for no sales for a particular month with no records. Is there anyway I can return a 0 instead. As you can see I tried "isNull to no avail.

Query==========>

SELECT Customer.Country, MetaDates.Month, MetaDates.Year, isnull(SUM(Order_Line_Invoice.Sales),0) AS Sales, SUM(Order_Line_Invoice.Cost) AS Cost,
SUM(Order_Line_Invoice.Sales) - SUM(Order_Line_Invoice.Cost) AS GM, 'Current 12 Months' AS yearNum


FROM Order_Line_Invoice INNER JOIN
Customer ON Order_Line_Invoice.CustId = Customer.CustId INNER JOIN
MetaDates ON Order_Line_Invoice.InvoiceDate = MetaDates.Date LEFT OUTER JOIN
Product ON Order_Line_Invoice.ProdId = Product.ProdId
WHERE (Customer.country = @GroupByFieldFilter) and (Order_Line_Invoice.InvoiceDate BETWEEN dateadd(m, datediff(m, 0, DATEADD(month,-11,@EndDate) ), 0) AND @EndDate)

GROUP BY MetaDates.Year, MetaDates.Month, Customer.country

UNION

SELECT Customer.Country , MetaDates.Month, MetaDates.Year, isnull(SUM(Order_Line_Invoice.Sales),0) AS Sales, SUM(Order_Line_Invoice.Cost) AS Cost,
SUM(Order_Line_Invoice.Sales) - SUM(Order_Line_Invoice.Cost) AS GM, 'Previous 12 Months' AS yearNum

FROM Order_Line_Invoice INNER JOIN
Customer ON Order_Line_Invoice.CustId = Customer.CustId INNER JOIN
MetaDates ON Order_Line_Invoice.InvoiceDate = MetaDates.Date LEFT OUTER JOIN
Product ON Order_Line_Invoice.ProdId = Product.ProdId
WHERE (Customer.country = @GroupByFieldFilter) and (Order_Line_Invoice.InvoiceDate BETWEEN dateadd(m, datediff(m, 0, DATEADD(month,-23,@EndDate) ), 0) AND dateadd(m, datediff(m, 0, DATEADD(month,-11,@EndDate) ), -1))

GROUP BY MetaDates.Year, MetaDates.Month, Customer.country
order by MetaDates.Year, MetaDates.Month asc

End Query=========>

Any help would be much appreciated.