SSRS SUM inside a Tablix column

Question

Hi,

I have a dataset that returns the following columns,

CategoryID, CategoryName, CategoryParentID, MonthIndex, Amount

I am adding this to a Matrix in SSRS 2008 and configuring for a recursive hierarchy which works fine. However, when I change the column from a

SUM(Fields!Amount.Value)

to a

SUM(Fields!Amount.Value, "GroupName", Recursive)

All of my month columns contain the same values - I am assuming because they all exist under the same scope?

Does anyone know how to perform a Recursive aggregate using a Matrix?

Thanks,