SSRS Totaling Issue With Mixed UOMs

I have run into an issue with our PackSlip. I am using the following code to total our gross weight:

=Switch(Lookup(Fields!PartNum.Value, Fields!PartNum.Value, Fields!IUM.Value, “Part”) = “KG” And Fields!Calc_DspLineShpQtyUom.Value = “LB”, formatnumber((Sum(Fields!bwGrossWeight_c.Value) * 2.20462), 2),

Lookup(Fields!PartNum.Value, Fields!PartNum.Value, Fields!IUM.Value, “Part”) = “LB” And Fields!Calc_DspLineShpQtyUom.Value = “KG”, formatnumber((Sum(Fields!bwGrossWeight_c.Value) / 2.20462), 2),

Lookup(Fields!PartNum.Value, Fields!PartNum.Value, Fields!IUM.Value, “Part”) = “LB” And Fields!Calc_DspLineShpQtyUom.Value = “LB”,formatnumber(Sum(Fields!bwGrossWeight_c.Value), 2),

Lookup(Fields!PartNum.Value, Fields!PartNum.Value, Fields!IUM.Value, “Part”) = “KG” And Fields!Calc_DspLineShpQtyUom.Value = “KG”,formatnumber(Sum(Fields!bwGrossWeight_c.Value), 2))

The pack slip is grouped at the order line level and has subtotals after each group. This works as expected. After all of the line groups there is a full order total at the bottom of the report. This works fine as long as everything on the Pack is the same UOM. If the first items on the pack are inventoried in LBs and sold in LBs but the second group of items is inventoried in KGs and sold in LBs then the second group’s Gross Weight does not get calculated.

It’s basically like the code is only looking at the first part to determine how to calculate everything instead of each individual line. If I could just total the subtotals somehow it would work as well.