Based on your description it is likely that you are only wiring a single data point to the "write to spreadsheet file". As Greg mentioned, post your vi so we can give you specific advice on your problem.
As a general point to get you started, to save a large number of data points, you should be wiring an array of values to "write to spreadsheet file". How you get this array entirely depends on how your code runs. If your data is being collected in a loop, you can either use an auto-indexed tunnel to build the array, or you can use an initialized array (say all zeros) of the appropriate size, replacing elements at each iteration and storing the changes in a shift register.
