We would probably need a sample workbook or at least screenshot of your viz with datasources and columns/rows shelves.
Did you try recreating same calculation? Perhaps, it uses some functions that are not supported by extracts.
Stephen Jay — When you extract a data source that includes calculations that reference blended data sources, those calculations will not be included in the extract. This is what I discovered when testing the behavior in Tableau 10.5 today.
I know this reply is really late to the game, but since I had the same question and was doing some tests today, I figured "better late than never".