Data disappears from cell after inputting info in 13 columns

[Isabellara]

Hi all,
To start with, I know very little about Excel, hence the need for help . My problem is this: a friend was given an Excel file she can use to input assignment results from her students, and it would automatically calculate percentages and averages. This file had only 10 columns for 10 assignments but she needed 15, so I copied formulas everywhere to extend it, and it seemed fine. However, now when you introduce data number 13 (not in column 13 but 13th assignment result regardless of the column) it calculates the percentages in that column but the data in the column with the average percentages for all assignments disappears. The formula for averaging is an IF formula
=IF(SUM(M6:BX6)=0,"",IF((O6+S6+W6+AA6+AE6+AI6+AM6+AQ6+AU6+AY6+BC6+BG6+BK6+BO6+BS6+BW6)>=1,"",(P6+T6+X6+AB6+AF6+AJ6+AN6+AR6+AV6+AZ6+BD6+BH6+BL6+BP6+BT6+BX6)/(O6+S6+W6+AA6+AE6+AI6+AM6+AQ6+AU6+AY6+BC6+BG6+BK6+BO6+BS6+BW6)))

I have Excel 2010. Not sure of the file Excel type but it may be 1997-2003?
I am not sure what data or formulas to show you all so you can help me.
In advance to anyone able to help, thank you very much for taking the time.
Thanks
Isabel

 

[JoeMo]

My guess is the formula is till there, but returning "" (zero-length string). Click on the cell to see if the formula is still there. If yes: go to Formulas>Formula Auditing>Evaluate Formula and step through the formula to see what's happening.