I have number of simple spread sheets that use the =sum command to add up a number of columns, or rows (e.g. =sum(B6:G6)). They work at the begining of the spread sheet, but stop working a few rows later. I have tried re-entering the formula and even re-wrote the spread sheet entirely, but a short time later the formulas stop working. I have already checked to make sure they are referencing the right cells. Any ideas?

Thank you.