I am having an issue with a spreadsheet.

Seems that some, not all the cells that have formulas are working.
When I created the spreadsheet I configured some cells to show the total of
some other cells it worked without any problems. Now when I open the
spreadsheet some of the configured cells will not work. By this I mean they
will not add up numbers from the other cells. This problem is intermittent.

Im using simple formulas such as =SUM(A1,B1)

How can I fix this?

Thanks in advance
Greg