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Tableau uses MS JET for Access, Excel, and text files, and particularly with Excel strange things can happen depending on what the cell formatting is. In the case you describe, I'm guessing that the calculated measures possibly have extra characters being returned that cause Tableau (via JET) to treat those fields as text on import, such that when you change the data type to number they are actually not valid numbers and therefore become Null.
One way to debug this is to export your Excel worksheet into .csv, then open it up in Notepad and inspect a few lines character by character. I've often found things like extra spaces at the beginning or end of fields this way.
If this doesn't help, I suggest posting packaged workbook (.twbx) with some sample data and someone here on the forums can help you get to the bottom of this.
Thanks Jonathan, that was it. Tableau was unable to recognize excel values formatted as '%'. I went back and changed my excel formatting to 'Number' and it worked. I have reported this issue and I suppose it will be logged as a bug somewhere.