Does anyone know how to get the data in a pivot table to auto-update when items in a list change?

I have a situation where there is a list of items (that changes with a drop-down menu). Some of the items in that list are duplicated and I need all repeated entries to be summed together. I followed the instructions at this link (below) and was able to get the result I want:

http://www.excelforum.com/excel-gene...-together.html

[... And kudos to Dave Petersen for giving the step-by-step on creating pivot tables! I've always shied away before.]

However, when I change my selection in the drop-down and the items in the list change, the pivot table retains the values from the first selection (when the pivot table was created) regardless of any selection made after that.

Can a pivot table update the fields when the source field data changes? The range for the data is the same, but the actual underlying data changes and the pivot table does update.

For reference, the range of the data does not change and the PivotTable Field list remains the same.