I have a very basic spreadsheet that has a simple formula that divides two
cell values to produce a percentage. The two values are the result of a sum
adding 20 other values.

I have recently found that if any of the 20 values are changed, the
resulting percentage formula does not update.

I have the spreadsheet set to AutoCalc and I ahve tried F9. Looking through
earlier posts on here I have discovered the Ctrl-Alt-F9 trick that forces a
full calculation. This seems to work okay and does update the formula. The
problem I have is that our Finance department are now questioning the
reliability of Excel. I don't want to have them pressing Ctrl-Alt-F9 every
20 minutes or so just to make sure the spreadsheet is updating.

Is this a known bug within Excel ? What is it that actually causes it to
happen ?
Is there a way to stop it happening ?

Any advice would be much appreciated.

Thanks

Martyn