Some of the numbers in the source data may be text, instead of numbers.
To convert them to real numbers, you can use one of the techniques shown
he

http://www.contextures.com/xlDataEntry03.html

wrote:
Please assist in identifying the reasons why the pivot table function within
Excel does not operate correctly. Several pieces of data are missing when
summed, but when you double click on the cell with an empty value it
correctly returns the spreadsheet with found data


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Debra Dalgleish
Excel FAQ, Tips & Book List
http://www.contextures.com/tiptech.html