Strange Excel file

I have a bespoke DOS based program which I now export data from in the form of Lotus workbook file(wk4). I then save this as an excel file. I am able to edit this file and save any changes I make. The problem I have is with working with formulas. If I take the sum of a column of figures, it adds it properly giving the correct figure. If I then go on to change a value of one of the figures in the column, nothing happens to the initial sum and it remains unchanged. I have also tried puting absolute references to the cell range in the formula and it still does not reflect any change in the final sum