EXCEL Subrotal Function

I have a normal spreadsheet with numeric data across 7 columns and 50 rows. The columns are for "CLASSES" and the rows are for "Years". At the bottom of the columns I have a 'Totals" row and I used the Subtotal function to sum the "CLASSES" since I will be using a filter later. That works fine. At the end of each row I used the SUM function for the "Year's Total". That works fine. However, at the bottom of the "Year's Total" I also used the Subtotal function but it returns a 0. Why?