I solved my problem. What I failed to say was that I have row groups as well. I needed to specify the child row group on the SUMs. (I would have thought it should be the parent row group)
=SUM (
IIF(
Sum(Fields!ColumnGroup1.Value,"Child") - Sum(Fields!ColumnGroup2.Value,"Child") > 0.00,
Sum(Fields!ColumnGroup1.Value,"Child") - Sum(Fields!ColumnGroup2.Value,"Child"),
CDec(0))
)
If you need to display a blank value for a subtotal of 0, use 'Nothing', if you need to display a value, then use 'CDec(0)'
Thanks