A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
The cells may be formatted as General or Number, but that doesn't mean they are numbers. Test them by writing the following formula:
=ISNUMBER(A2)
Where A2 is one of the numbers in the column that aren't summing. If this says FALSE the cell isn't a number.
Solution: select a blank cell that has a general format and choose Copy. Select the column you want to sum and choose Paste, Paste Special, Add, OK.