Without digging into your SSRS report I am not really sure; however, I did notice that the first two objects in your lookup were the same "Fields!Item_Number.value" usually, or at least in the cases that I have seen they are different like "=Lookup(Fields!SaleProdId.Value, Fields!ProductID.Value, Fields!Name.Value, "Product")"
But then I could be totally off base, please let me know how it goes,