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I am trying to use a very simple formula which is =SUM(B9:B11). However the cell doesn't compute for some reason.

I've used Excel for years and have never had this problem. Any idea why it may be failing to update the SUM?

I'm using Excel 2007 on Windows 7 Pro. I've opened the spreadsheet on multiple different machines with the same results so it appears to be an issue with the spreadsheet itself and not Excel or the computer.

Additional Note: If I recreate the =SUM formula it will recompute the total. However, if I change one of the number it still doesn't auto-recalculate.

Also, if I press `F9` the SUM will recalculate being manually forced to.