Subtotal function is not working correctly
 

I recently got a new computer at wortk with Excel 2003. My previous computer
had an older version, and the subtotal function worked fine. Here's my
problem. A report that I work on needs to be sorted 3 different ways, first
by branch office, Billing Type, then System Type. I then subtotal by branch
as my first level. Then I subtotal by Billing Type, in which I uncheck the
replace current subtotals. I then do the same thing with the Sytem Type.
The first level always works fine. Once I get to Billing Type, it starts
screwing up. It seems that if there is a blank cell in the mix, or
something out of the ordinary, it messes up my subtotalling. Any ideas on
how to correct this?